Erlang forex

Erlang forex
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Um erro com ets e leitura de arquivos.
Disclaimer: Eu não escrevi este código, estou apenas tentando fazê-lo funcionar.
Estou tentando obter o código a partir daqui a trabalhar. A configuração é uma máquina Ubuntu 14.04 de 64 bits com o Erlang instalado.
A sequência de ações é a seguinte: O que estou fazendo é o seguinte:
clone o código cd na pasta com o código e erl no terminal make: all ([load]). polis: create (). polis: start (). benchmarker: start (slidingwindow50).
Os erros que recebo são:
Revisar o código do fx. erl mostra que o problema provavelmente decorre da parte ets e um arquivo. txt não sendo lido do disco e em uma tabela na memória.
Atualização: Estive tentando investigar a função de fx. Até agora, eu tive tais resultados: fx: sim (qualquer coisa) saídas "Iniciado" e faz com que o shell não responda; fx: init (). resulta em.
Inicializando tabelas de moeda FX: [metadados, 'EURUSD15', 'EURUSD30', 'EURUSD60'] FX metadata & amp; tabelas de moeda inicializadas e escritas em arquivo. Está bem.
fx: loop (). produz muitos.
Novo registro inserido na tabela: 'EURUSD15'
com a última linha sendo.
Nova atualização do FOREX_DB começando com:
Correndo ets: i (). depois de fx: init (). não mostra tabela chamada EURUSD15.
Por que isso pode ser, e como posso corrigir isso?
P. S. Se uma solução de trabalho for encontrada, pretendo pedir que as alterações sejam mescladas com o código original (basicamente, vou tentar uma solicitação de envio).
Estou familiarizado com essa plataforma, e estamos executando isso em nossas máquinas, nós a usamos como nosso material de treinamento no ano passado.
Isso está ocorrendo porque você não iniciou o simulador forex antes de iniciar o benchmarker.

Este artigo descreve a unidade de telecomunicações Erlang, e sua aplicação na teoria do teletrafico. Online Erlang Traffic Calculators também são hospedados neste site e podem ser usados ​​gratuitamente.
Destaques do site.
VoIP.
Terminação de qualidade premium para operadoras de atacado com sua própria opção VoIP.
Nossas taxas mais baixas para os atacadistas com sua própria opção VoIP.
Nosso exclusivo sistema de cobrança de callshop online.
Erlang - uma unidade de tráfego.
Por exemplo, se um grupo de usuários fez 30 chamadas em uma hora, e cada chamada teve uma duração média de chamada de 5 minutos, então o número de Erlangs que isso representa é elaborado da seguinte maneira:
As medidas de tráfego de Erlang são feitas para ajudar os designers de redes de telecomunicações a entender os padrões de tráfego dentro de suas redes de voz. Isso é essencial para que eles criem com sucesso sua topologia de rede e estabeleçam os tamanhos de grupo de troncos necessários.
As medidas ou estimativas do tráfego de Erlang podem ser usadas para determinar quantas linhas são necessárias entre um sistema de telefone e um escritório central (linhas de troca PSTN) ou entre vários locais de rede.
Modelos de tráfego Erlang.
O principal modelo de tráfego de Erlang está listado abaixo, com links para as calculadoras on-line gratuitas neste site: Erlang B.
Leitura adicional.
A. K. Erlang - um tributo.
Em 1909, ele publicou seu primeiro trabalho: The Theory of Probabilities and Phone Conversations. Ele ganhou reconhecimento mundial por seu trabalho, e sua fórmula foi aceita para uso pelo General Post Office no Reino Unido.
Erlang nunca se casou. Ele trabalhou para a Copenhagen Telephone Company por vinte anos, até sua morte em 1929. Durante a década de 1940, o Erlang tornou-se a unidade aceita de medição do tráfego de telecomunicações, e sua fórmula ainda é usada hoje no projeto das modernas redes de telecomunicações.
Conclusão.
Por que não tentar o nosso Telecom Design Forum? É um grupo de notícias interativo que você pode usar para trocar idéias sobre design de telecomunicações e nossos produtos.

QUANTLABS.
Quant Resources for Traders.
Neuroevolution Through Erlang PDF agora existe & # 8230; aprendizado de evolução para negociação de forex e previsão de mercado.
Neuroevolution Through Erlang PDF agora existe & # 8230; aprendizado de evolução para negociação de forex e previsão de mercado.
Neuroevolution Through Erlang PDF agora existe. Eu, pessoalmente, não passaria muito tempo com isso devido a uma alta probabilidade de falha no longo prazo. Esta é uma tarefa difícil, que é apenas recomendada para pessoas altamente avançadas que têm as costeletas de tecnologia e matemática.
Obrigado por enviar de Sholom.
OBSERVAÇÃO Eu agora posto minhas ALERTAS DE NEGOCIAÇÃO na minha conta pessoal de FACEBOOK e TWITTER. Não se preocupe porque não publico vídeos de gato estúpidos ou o que eu como!

Erlang forex
Este módulo contém funções de correspondência de expressões regulares para strings e binários.
A sintaxe de expressão regular e a semântica se assemelham à de Perl.
Os algoritmos de correspondência da biblioteca são baseados na biblioteca do PCRE, mas nem toda a biblioteca do PCRE está interativa e algumas partes da biblioteca vão além do que o PCRE oferece. Atualmente, a PCRE versão 8.40 (data de lançamento 2017-01-11) é usada. As seções da documentação PCRE relevantes para este módulo estão incluídas aqui.
A sintaxe literal Erlang para strings usa o caractere "\" (barra invertida) como um código de escape. Você precisa escapar backslashes em cordas literais, tanto no seu código como no shell, com uma barra invertida adicional, ou seja, "\\".
Tipos de dados.
Tipo de dados opaco contendo uma expressão regular compilada. O mp () é garantido para ser uma tupla () tendo o átomo re_pattern como seu primeiro elemento, para permitir a correspondência em guardas. A aridez da tupla ou o conteúdo dos outros campos podem mudar no futuro Erlang / OTP lançamentos.
Ângulo Unicode |
                                       Â
Ñ ​​В В sem castigo |
Dólar de dólar
В В В extended |
ВВВВВ firstline |
В В В multiline |
В В В no au au c c c c c ture ture ture ture ture ture ture
Bónus de duplos |
Ângulo ungreedy |
Bsr_anycrlf |
Bsr_unicode |
Nao_start_optimize |
Nunca_utf.
O retorno desta função é uma string com a versão PCRE do sistema que foi usada na compilação Erlang / OTP.
O mesmo que compilação (Regexp, [])
Compila uma expressão regular, com a sintaxe descrita abaixo, em um formato interno a ser usado mais tarde como parâmetro para executar / 2 e executar / 3.
Compilar a expressão regular antes da correspondência é útil se a mesma expressão for usada na correspondência contra vários assuntos durante a vida útil do programa. Compilar uma vez e executar muitas vezes é muito mais eficiente do que compilar cada vez que alguém quer combinar.
Quando a opção unicode é especificada, a expressão regular deve ser especificada como uma lista de caracteres unicode válida (), caso contrário, como qualquer iodata válido ().
A expressão regular é especificada como um charlist Unicode () e o código de expressão regular resultante deve ser executado contra um assunto de charlist () Unicode válido. Considere também a opção ucp ao usar caracteres Unicode.
O padrão é forçado a ser "ancorado", isto é, é obrigado a combinar apenas no primeiro ponto de correspondência na seqüência que é pesquisada (a "seqüência de caracteres"). Este efeito também pode ser alcançado por construções apropriadas no próprio padrão.
As letras no padrão correspondem letras maiúsculas e minúsculas. É equivalente à opção Perl / i e pode ser alterado dentro de um padrão por uma configuração de opção (? I). As letras maiúsculas e minúsculas são definidas como no conjunto de caracteres ISO 8859-1.
Um dólar de metacaracteres no padrão coincide apenas no final da seqüência do assunto. Sem essa opção, um dólar também coincide imediatamente antes de uma nova linha no final da string (mas não antes de outras linhas novas). Esta opção é ignorada se a opção multilinha for especificada. Não há nenhuma opção equivalente em Perl, e não pode ser configurada dentro de um padrão.
Um ponto no padrão corresponde a todos os caracteres, incluindo aqueles que indicam a linha nova. Sem ele, um ponto não coincide quando a posição atual está em uma nova linha. Esta opção é equivalente à opção Perl / s e pode ser alterada dentro de um padrão por uma configuração de opção (? S). Uma classe negativa, como [^ a], coincide sempre com os caracteres da nova linha, independentemente da configuração desta opção.
Se esta opção estiver definida, a maioria dos caracteres de espaço em branco no padrão são totalmente ignorados, exceto quando escapados ou dentro de uma classe de personagem. No entanto, o espaço em branco não é permitido dentro de sequências como (? & Gt; que introduzem vários subpatrones entre parênteses, nem dentro de um quantificador numérico, como. No entanto, é permitido espaço branco ignorável entre um item e um quantificador seguinte e entre um quantificador e um seguimento + que indica possessividade.
O espaço em branco não usou para incluir o caractere VT (código 11), porque Perl não tratou este caracter como espaço em branco. No entanto, Perl mudou na versão 5.18, então o PCRE seguiu na versão 8.34, e VT agora é tratado como espaço em branco.
Isso também faz com que os caracteres entre um # não escondido fora de uma classe de caracteres e a próxima nova linha, inclusive, sejam ignorados. Isso é equivalente à opção Perl / x, e pode ser alterado dentro de um padrão por uma configuração de opção (? X).
Com esta opção, os comentários dentro de padrões complicados podem ser incluídos. No entanto, observe que isso se aplica apenas a caracteres de dados. Os caracteres de espaço em branco nunca podem aparecer dentro de seqüências de caracteres especiais em um padrão, por exemplo dentro da seqüência (? (Que introduz um subpattern condicional.
É necessário um padrão não dividido para coincidir antes ou na primeira linha nova na seqüência do assunto, embora o texto correspondente possa continuar por cima da nova linha.
Por padrão, PCRE trata a seqüência de caracteres de assunto como consistindo de uma única linha de caracteres (mesmo que contenha novas linhas). O metacaracter do "início da linha" (^) coincide apenas no início da seqüência de caracteres, enquanto o metacaracter ($) de "fim da linha" corresponde apenas ao final da string ou antes de uma nova linha de encerramento (a menos que a opção dollar_endonly seja especificada ). Isso é o mesmo que no Perl.
Quando esta opção é especificada, as construções de "início de linha" e "fim de linha" correspondem imediatamente a seguir ou imediatamente antes das novas linhas internas na seqüência de assunto, respectivamente, bem como no início e no final. Isso é equivalente à opção Perl / m e pode ser alterado dentro de um padrão por uma configuração de opção (? M). Se não houver linhas novas em uma seqüência de assunto, ou nenhuma ocorrência de ^ ou $ em um padrão, a definição de multilinha não tem efeito.
Desativa o uso de parênteses de captura numerados no padrão. Algum parêntesis de abertura que não é seguido? se comporta como se fosse seguido por: Os parênteses com nomes podem ainda ser usados ​​para capturar (e eles adquirem números da maneira usual). Não existe uma opção equivalente no Perl.
Os nomes usados ​​para identificar subpastas de captura não precisam ser únicos. Isso pode ser útil para certos tipos de padrões quando se sabe que apenas uma instância do subpattern nomeado pode ser igualada. Mais detalhes sobre subpatterns nomeados são fornecidos abaixo.
Inverte a "ganância" dos quantificadores para que eles não sejam gananciosos por padrão, mas se tornem gananciosos se seguidos por "?". Não é compatível com Perl. Também pode ser definido por uma configuração de opção (? U) dentro do padrão.
Substitui a definição padrão de uma nova linha na seqüência de assunto, que é LF (ASCII 10) em Erlang.
Newline é indicado por um único personagem cr (ASCII 13).
Newline é indicado por um único caractere LF (ASCII 10), o padrão.
Newline é indicado pela sequência CRLF de dois caracteres (ASCII 13 seguida por ASCII 10).
Qualquer uma das três seqüências precedentes deve ser reconhecida.
Qualquer uma das seqüências de nova linha acima e as seqüências de Unicode VT (guia vertical, U + 000B), FF (formfeed, U + 000C), NEL (linha seguinte, U + 0085), LS (separador de linha, U + 2028) e PS (separador de parágrafo, U + 2029).
Especifica especificamente que \ R é para coincidir apenas com as seqüências CR, LF ou CRLF, e não com os caracteres da linha nova específicos do Unicode.
Especifica especificamente que \ R é para combinar todos os caracteres da linha nova Unicode (incluindo CRLF, e assim por diante, o padrão).
Desativa a otimização que pode funcionar mal se "itens especiais de início de padrão" estiverem presentes na expressão regular. Um exemplo típico seria ao combinar "DEFABC" contra "(* COMMIT) ABC", onde a otimização de inicialização do PCRE ignoraria o assunto até "A" e nunca perceberia que a instrução (* COMMIT) deveria ter feito a correspondência falhou. Esta opção só é relevante se você usar "itens de início de padrão", conforme discutido na seção Detalhes da Expressão Regular do PCRE.
Especifica que as propriedades de caracteres Unicode devem ser usadas ao resolver \ B, \ b, \ D, \ d, \ S, \ s, \ W e \ w. Sem esta bandeira, apenas as propriedades ISO Latin-1 são usadas. O uso de propriedades Unicode prejudica o desempenho, mas é semanticamente correto ao trabalhar com caracteres Unicode além do intervalo ISO Latin-1.
Especifica que o (* UTF) e / ou (* UTF8) "itens de início de padrão" são proibidos. Esta bandeira não pode ser combinada com a opção unicode. Útil se os padrões ISO Latin-1 de uma fonte externa forem compilados.
Obtém uma expressão regular compilada e um item e retorna os dados relevantes da expressão regular. O único item suportado é namelist, que retorna a tupla, contendo os nomes de todos os subpatrones nomeados (únicos) na expressão regular. Por exemplo:
Observe no segundo exemplo que o nome duplicado só ocorre uma vez na lista retornada e que a lista está em ordem alfabética, independentemente de onde os nomes estão posicionados na expressão regular. A ordem dos nomes é igual à ordem das subexpressões capturadas se for especificado como uma opção para executar / 3. Você pode, portanto, criar um mapeamento de nome para valor a partir do resultado da execução / 3, assim:
Mesmo substituindo (Assunto, RE, Substituição, []).
                                       Â
В В В В notempty |
В В В В В В В В В В В В st st st st st
Bsr_anycrlf |
Bsr_unicode |
CompileOpt.
Substitui a parte correspondente da seqüência Assunto com o conteúdo de Reposição.
As opções permitidas são as mesmas que para a execução / 3, exceto que a captura da opção não é permitida. Em vez disso, está presente. O tipo de retorno padrão é iodata, construído de forma a minimizar a cópia. O resultado iodata pode ser usado diretamente em muitas operações de E / S. Se for desejada uma lista plana (), especifique. Se um binário for desejado, especifique.
Como na função run / 3, um mp () compilado com a opção unicode requer Assunto para ser um charlist Unicode (). Se a compilação for feita de forma implícita e a opção de compilação unicode for especificada para esta função, tanto a expressão regular como o Assunto são especificados como charlist () s. Unicode válido.
A seqüência de substituição pode conter o caractere especial & amp; , que insere toda a expressão correspondente no resultado, e a seqüência especial \ N (onde N é um número inteiro & gt; 0), \ g N ou \ g, resultando no número de subexpressão N, é inserido no resultado. Se nenhuma subexpressão com esse número é gerada pela expressão regular, nada é inserido.
Para inserir um & amp; ou um \ no resultado, precedê-lo com um \. Observe que Erlang já dá um significado especial para \ em cordas literais, então um único \ deve ser escrito como "\\" e, portanto, um duplo \ como "\\\\".
Tal como acontece com o run / 3, os erros de compilação aumentam a exceção de Badarg. compile / 2 pode ser usado para obter mais informações sobre o erro.
O mesmo que executar (Assunto, RE, []).
В В В В | jogo | nomatch |
                                       Â
В В В В notempty |
В В В В В В В В В В В В st st st st st
'ReportWerrors' |
Bsr_anycrlf |
Bsr_unicode |
CompileOpt.
Todos | todos | all_but_first | all_names | primeiro | nenhum | ValueList.
В В В | ListConversionData | binário()
В В В match_limit | match_limit_recursion |
Executa uma correspondência de expressões regulares e retorna match / or nomatch. A expressão regular pode ser especificada como iodata (), caso em que é compilado automaticamente (como por compilação / 2) e executado, ou como um mp précompilado, caso em que é executado diretamente contra o assunto.
Quando a compilação está envolvida, o erro de exceção é lançado se ocorrer um erro de compilação. Chamar compilação / 2 para obter informações sobre a localização do erro na expressão regular.
Se a expressão regular for previamente compilada, a lista de opções só pode conter as seguintes opções:
Caso contrário, todas as opções válidas para a compilação de função / 2 também são permitidas. Opções permitidas tanto para compilação e execução de uma correspondência, ou seja, ancoradas e, afetam tanto a compilação quanto a execução se presentes junto com uma expressão regular não pré-compilada.
Se a expressão regular foi previamente compilada com a opção unicode, o assunto deve ser fornecido como um charlist Unicode válido (), caso contrário qualquer iodata () fará. Se a compilação estiver envolvida e a opção unicode for especificada, tanto o Assunto como a expressão regular devem ser especificados como Listas de Unicode válidas ().
/ define o que retornar da função após uma correspondência bem-sucedida. A tupla de captura pode conter uma especificação de valor, indicando quais das substrings capturadas devem ser retornadas e uma especificação de tipo, informando como as substrings capturadas devem ser retornadas (como tuplas, listas ou binários do índice). As opções são descritas em detalhes abaixo.
Se as opções de captura descrevem que nenhuma captura de substring deve ser feita (), a função retorna a combinação de átomos únicos após uma correspondência bem sucedida, caso contrário, a tupla. Desativar a captura pode ser feito especificando nenhum ou uma lista vazia como ValueSpec.
Option report_errors acrescenta a possibilidade de que uma tupla de erro seja retornada. A tupla indica um erro correspondente (match_limit ou match_limit_recursion), ou um erro de compilação, onde a tupla de erro tem o formato>. Observe que, se a opção report_errors não for especificada, a função nunca retornará as tuplas de erro, mas relata erros de compilação como uma exceção incorreta e combinações falhadas por causa de limites de correspondência excedidos simplesmente como o nômade.
As seguintes opções são relevantes para a execução:
Os limites são executados / 3 para coincidir na primeira posição correspondente. Se um padrão foi compilado com ancorado, ou acabou por ser ancorado em virtude de seu conteúdo, ele não pode ser descompactado em tempo de correspondência, portanto, não há opção não cortada.
Implementa pesquisa global (repetitiva) (flag g em Perl). Cada correspondência é retornada como uma lista separada () que contém a correspondência específica e as subexpressões correspondentes (ou conforme especificado pela captura de opções. A parte Capturada do valor de retorno é, portanto, uma lista () da lista () s quando esta opção é especificada.
A interação da opção global com uma expressão regular que corresponde a uma string vazia surpreende alguns usuários. Quando a opção global é especificada, execute / 3 lida com correspondências vazias da mesma maneira que Perl: uma partida de comprimento zero em qualquer ponto também é tentada novamente com as opções [ancorado, notempty_atstart]. Se essa pesquisa der um resultado do comprimento & gt; 0, o resultado está incluído. Exemplo:
As seguintes combinações são realizadas:
A expressão regular (| at) primeiro jogo na posição inicial de string cat, dando o conjunto de resultados [,] (o segundo é devido à subexpressão marcada pelos parênteses). Como o comprimento da partida é 0, não avançamos para a próxima posição ainda.
No deslocamento 0 com [ancorado, notempty_atstart]
A busca é tentada novamente com as opções [ancoradas, notempty_atstart] na mesma posição, que não dão resultados interessantes de comprimento maior, de modo que a posição de busca seja avançada para o próximo caractere (a).
A pesquisa resulta em [,], então esta pesquisa também é repetida com as opções extras.
No deslocamento 1 com [ancorado, notempty_atstart]
Ab alternante é encontrado e o resultado é [,]. O resultado é adicionado à lista de resultados e a posição na cadeia de pesquisa é avançada duas etapas.
A pesquisa mais uma vez corresponde à string vazia, dando [,].
No deslocamento 1 com [ancorado, notempty_atstart]
Isso não dá resultado de comprimento & gt; 0 e estamos na última posição, então a pesquisa global está completa.
O resultado da chamada é:
Uma string vazia não é considerada uma correspondência válida se esta opção for especificada. Se existem alternativas no padrão, elas são tentadas. Se todas as alternativas combinarem com a seqüência vazia, a partida completa falhará.
Se o padrão a seguir for aplicado a uma seqüência de caracteres que não comece com "a" ou "b", ele normalmente combinaria a seqüência vazia no início do assunto:
Com a opção notempty, esta partida é inválida, então execute / 3 procure mais na string para ocorrências de "a" ou "b".
Como notempty, exceto que uma correspondência de string vazia que não está no início do assunto é permitida. Se o padrão estiver ancorado, tal correspondência só pode ocorrer se o padrão contiver \ K.
Perl não tem equivalente direto a notempty ou notempty_atstart, mas faz um caso especial de uma combinação padrão da cadeia vazia dentro da função split () e ao usar o modificador / g. O comportamento de Perl pode ser emulado depois de combinar uma string nula primeiro tentando a correspondência novamente no mesmo deslocamento com notempty_atstart e ancorado e, se isso falhar, avançando o deslocamento inicial (veja abaixo) e tentando uma correspondência normal novamente.
Especifica que o primeiro caractere da seqüência de assunto não é o início de uma linha, portanto, o metacaractere circunflexo não deve coincidir com ela. Definir isso sem multilinha (em tempo de compilação) faz com que o circunflexo nunca corresponda. Esta opção apenas afeta o comportamento do metacaractere circunflexo. Não afeta \ A.
Especifica que o fim da seqüência do assunto não é o fim de uma linha, portanto, o metacaracterismo do dólar não é para combiná-lo nem (exceto no modo multilinha) uma nova linha imediatamente antes dele. Definir isso sem multilinha (no tempo de compilação) faz com que o dólar nunca corresponda. Essa opção afeta apenas o comportamento do metacaracterismo do dólar. Não afeta \ Z ou \ z.
Dá melhor controle sobre o tratamento de erros no run / 3. Quando especificado, os erros de compilação (se a expressão regular ainda não estiver compilada) e os erros de tempo de execução são explicitamente retornados como uma tupla de erro.
Os seguintes são os possíveis erros de tempo de execução:
A biblioteca PCRE define um limite em quantas vezes a função de correspondência interna pode ser chamada. Padrões para 10.000.000 na biblioteca compilada para Erlang. Se for retornado, a execução da expressão regular atingiu esse limite. Normalmente, isso deve ser considerado como um nômade, que é o valor de retorno padrão quando isso ocorre, mas, especificando report_errors, você está informado quando a partida falhar devido a muitas chamadas internas.
Este erro é muito parecido com match_limit, mas ocorre quando a função de correspondência interna do PCRE é "recursivamente" chamada mais vezes do limite de match_limit_recursion, que por padrão é de 10.000.000. Observe que, enquanto os valores de match_limit e match_limit_default forem mantidos nos valores padrão, o erro match_limit_recursion não pode ocorrer, pois o erro match_limit ocorre antes disso (cada chamada recursiva também é uma chamada, mas não inversamente). Ambos os limites podem, no entanto, ser alterados, quer através da definição de limites diretamente na seqüência de expressões regulares (veja a seção Detalhes da Ejetão Regular PCRE) ou especificando opções para executar / 3.
É importante entender que o que é referido como "recursão" ao limitar as correspondências não é recursão na pilha C da máquina Erlang ou na pilha de processo Erlang. A versão PCRE compilada na VM Erlang usa memória de "heap" de máquina para armazenar valores que devem ser mantidos ao longo da recursão em correspondências de expressão regular.
Limita o tempo de execução de uma partida de uma maneira específica da implementação. É descrito a seguir pela documentação PCRE:
Isso significa que as correspondências de expressão regular fugas podem falhar mais rápido se o limite for reduzido usando esta opção. O valor padrão 10,000,000 é compilado na VM Erlang.
Esta opção não afeta de modo algum a execução da VM Erlang em termos de "BIFs de longa duração". executar / 3 sempre dá controle de volta para o agendador dos processos Erlang em intervalos que garantem as propriedades em tempo real do sistema Erlang.
Limita o tempo de execução eo consumo de memória de uma partida de uma maneira específica da implementação, muito semelhante ao match_limit. É descrito a seguir pela documentação PCRE:
O Erlang VM usa uma biblioteca PCRE onde a memória heap é usada quando ocorre recursão de correspondência de expressão regular. Isso, portanto, limita o uso da pilha da máquina, não da pilha C.
Especificar um valor menor pode resultar em correspondências com falhas de recursão profunda, quando devem ter correspondido:
Esta opção e opção match_limit são apenas usadas em casos raros. É recomendável entender os internals da biblioteca PCRE antes de adulterar esses limites.
Comece a combinar no deslocamento (posição) especificado na seqüência do assunto. O deslocamento é baseado em zero, de modo que o padrão é (toda a seqüência do assunto).
Substitui a definição padrão de uma nova linha na seqüência de assunto, que é LF (ASCII 10) em Erlang.
Newline é indicado por um único caractere CR (ASCII 13).
Newline é indicado por um único caractere LF (ASCII 10), o padrão.
Newline é indicado pela sequência CRLF de dois caracteres (ASCII 13 seguida por ASCII 10).
Qualquer uma das três seqüências precedentes é reconhecida.
Qualquer uma das seqüências de nova linha acima e as seqüências de Unicode VT (guia vertical, U + 000B), FF (formfeed, U + 000C), NEL (linha seguinte, U + 0085), LS (separador de linha, U + 2028) e PS (separador de parágrafo, U + 2029).
Especifica especificamente que \ R é para coincidir apenas com as sequências CR LF, ou CRLF, e não com os caracteres da linha nova específicos do Unicode. (Substitui a opção de compilação.)
Especifica especificamente que \ R é para combinar todos os caracteres da linha nova Unicode (incluindo CRLF, e assim por diante, o padrão). (Substitui a opção de compilação.)
Especifica quais substrings capturadas são retornadas e em que formato. Por padrão, executar / 3 captura toda a parte correspondente da substring e todas as subpastas de captura (todo o padrão é automaticamente capturado). O tipo de retorno padrão é (zero-based) índices das partes capturadas da string, especificadas como pares (o índice tipo de captura).
Como um exemplo do comportamento padrão, a seguinte chamada retorna, como primeira e única corda capturada, a parte correspondente do assunto ("abcd" no meio) como um par de índice, onde as posições de caracteres são baseadas em zero, assim como em compensações:
O valor de retorno desta chamada é:
Outro (e bastante comum) caso é onde a expressão regular corresponde a todo o assunto:
Aqui, o valor de retorno indica, de forma correspondente, toda a cadeia, começando no índice 0 e tem 10 caracteres:
Se a expressão regular contiver subpastas de captura, como em:
Todo o assunto correspondente é capturado, bem como as substrings capturadas:
O padrão de correspondência completo sempre dá o primeiro valor de retorno na lista e os subpatrones restantes são adicionados na ordem em que ocorreram na expressão regular.
A tupla de captura é compilada da seguinte maneira:
Especifica quais padrões (sub) capturados devem ser retornados. ValueSpec pode ser um átomo que descreva um conjunto predefinido de valores de retorno ou uma lista contendo os índices ou os nomes de subpatrones específicos para retornar.
A seguir estão os conjuntos predefinidos de subpatterns:
Todos os subpatterns capturados, incluindo a corda de correspondência completa. Este é o padrão.
Todos os subpatrones nomeados na expressão regular, como se uma lista () de todos os nomes em ordem alfabética fosse especificada. A lista de todos os nomes também pode ser recuperada com inspeção / 2.
Somente o primeiro subpattern capturado, que é sempre a parte completa do assunto. Todos os subpatrones explicitamente capturados são descartados.
Todos, exceto o primeiro subpattern correspondente, ou seja, todos os subpatrones explicitamente capturados, mas não a parte completa correspondente da seqüência de assunto. Isso é útil se a expressão regular como um todo corresponde a uma grande parte do assunto, mas a parte em que você está interessado está em um subpattern explicitamente capturado. Se o tipo de retorno for listado ou binário, não retornar subpatrones que você não está interessado é uma boa maneira de otimizar.
Não retorna subpatrones correspondentes, dá a combinação de átomos únicos como o valor de retorno da função ao combinar com sucesso em vez do retorno. Especificar uma lista vazia dá o mesmo comportamento.
A lista de valores é uma lista de índices para que os subpatrones retornem, onde o índice 0 é para todo o padrão e 1 é para o primeiro subpattern de captura explícita na expressão regular, e assim por diante. Ao usar subpatrones capturados nomeados (veja abaixo) na expressão regular, pode-se usar átomo () s ou string () s para especificar os subpatrones a serem retornados. Por exemplo, considere a expressão regular:
combinado contra a string "ABCabcdABC", capturando apenas a parte "abcd" (o primeiro subpattern explícito):
A chamada dá o seguinte resultado, pois o primeiro subpattern explicitamente capturado é "(abcd)", correspondendo "abcd" no assunto, na posição (baseada em zero) 3, do comprimento 4:
Considere a mesma expressão regular, mas com o subpattern explicitamente chamado 'FOO':
Com esta expressão, ainda podemos dar o índice do subpattern com a seguinte chamada:
dando o mesmo resultado que antes. Mas, como o subpattern é chamado, também podemos especificar seu nome na lista de valores:
Isso daria o mesmo resultado que os exemplos anteriores, a saber:
A lista de valores pode especificar índices ou nomes não presentes na expressão regular, caso em que os valores de retorno variam dependendo do tipo. Se o tipo for índice, a tupla é retornada para valores sem subpattern correspondente na expressão regular, mas para os outros tipos (binário e lista), os valores são o binário vazio ou a lista, respectivamente.
Opcionalmente, especifica como as substrings capturadas devem ser retornadas. Se for omitido, o padrão do índice é usado.
Tipo pode ser um dos seguintes:
Retorna substrings capturadas como pares de índices de bytes na cadeia de assunto e comprimento da string correspondente no assunto (como se a seqüência de assunto fosse achatada com erlang: iolist_to_binary / 1 ou unicode: characters_to_binary / 2 antes da correspondência). Observe que a opção unicode resulta em índices orientados a bytes em um binário codificado UTF-8 (possivelmente virtual). Uma tupla de índice de bytes pode, portanto, representar um ou dois caracteres quando umicode está em vigor. Isso pode parecer contra intuitivo, mas foi considerado a maneira mais eficaz e útil de fazê-lo. Para retornar listas, em vez disso, pode resultar em um código mais simples se isso for desejado. Este tipo de retorno é o padrão.
Retorna equivalências correspondentes como listas de caracteres (Erlang string () s). A opção unicode é usada em combinação com a seqüência \ C na expressão regular, um subpattern capturado pode conter bytes que não são válidos UTF-8 (\ C coincide com bytes, independentemente da codificação de caracteres). Nesse caso, a captura da lista pode resultar nos mesmos tipos de tuplas que unicode: characters_to_list / 2 pode retornar, ou seja, três tuplas com etiqueta incompleta ou erro, os caracteres convertidos com sucesso e o inválido UTF-8 tail da conversão como um binário . A melhor estratégia é evitar o uso da seqüência \ C ao capturar listas.
Retorna correspondência de substrings como binários. Se a opção unicode for usada, esses binários estão em UTF-8. Se a seqüência \ C for usada junto com unicode, os binários podem ser UTF-8 inválidos.
Em geral, os subpatrones que não receberam um valor na partida são retornados como a tupla quando o tipo é índice. Os subpatrones não atribuídos são retornados como o binário vazio ou lista, respectivamente, para outros tipos de retorno. Considere a seguinte expressão regular:
Existem três subpatrones de captura explícita, onde a posição de parêntese de abertura determina a ordem no resultado, portanto ((? & Lt; FOO & gt; abdd) | a (... d)) é índice subpattern 1, (? & Lt; FOO & gt; abdd) é o índice subpattern 2, e (..d) é o índice subpattern 3. Quando comparado com a seguinte string:
o subpattern no índice 2 não coincide, pois "abdd" não está presente na string, mas o padrão completo corresponde (por causa da alternativa a (... d)). The subpattern at index 2 is therefore unassigned and the default return value is:
Setting the capture Type to binary gives:
Here the empty binary ( <<>> ) represents the unassigned subpattern. In the binary case, some information about the matching is therefore lost, as <<>> can also be an empty string captured.
If differentiation between empty matches and non-existing subpatterns is necessary, use the type index and do the conversion to the final type in Erlang code.
When option global is speciified, the capture specification affects each match separately, so that:
For a descriptions of options only affecting the compilation step, see compile/2 .
Same as split(Subject, RE, []) .
В В В В anchored |
В В В В notempty |
В В В В notempty_atstart |
В В В В bsr_anycrlf |
В В В В bsr_unicode |
В В В В CompileOpt.
Splits the input into parts by finding tokens according to the regular expression supplied. The splitting is basically done by running a global regular expression match and dividing the initial string wherever a match occurs. The matching part of the string is removed from the output.
As in run/3 , an mp() compiled with option unicode requires Subject to be a Unicode charlist() . If compilation is done implicitly and the unicode compilation option is specified to this function, both the regular expression and Subject are to be specified as valid Unicode charlist() s.
The result is given as a list of "strings", the preferred data type specified in option return (default iodata ).
If subexpressions are specified in the regular expression, the matching subexpressions are returned in the resulting list as well. Por exemplo:
The text matching the subexpression (marked by the parentheses in the regular expression) is inserted in the result list where it was found. This means that concatenating the result of a split where the whole regular expression is a single subexpression (as in the last example) always results in the original string.
As there is no matching subexpression for the last part in the example (the "g"), nothing is inserted after that. To make the group of strings and the parts matching the subexpressions more obvious, one can use option group , which groups together the part of the subject string with the parts matching the subexpressions when the string was split:
Here the regular expression first matched the "l", causing "Er" to be the first part in the result. When the regular expression matched, the (only) subexpression was bound to the "l", so the "l" is inserted in the group together with "Er". The next match is of the "n", making "a" the next part to be returned. As the subexpression is bound to substring "n" in this case, the "n" is inserted into this group. The last group consists of the remaining string, as no more matches are found.
By default, all parts of the string, including the empty strings, are returned from the function, for example:
as the matching of the "g" in the end of the string leaves an empty rest, which is also returned. This behavior differs from the default behavior of the split function in Perl, where empty strings at the end are by default removed. To get the "trimming" default behavior of Perl, specify trim as an option:
The "trim" option says; "give me as many parts as possible except the empty ones", which sometimes can be useful. You can also specify how many parts you want, by specifying :
Notice that the last part is "ang", not "an", as splitting was specified into two parts, and the splitting stops when enough parts are given, which is why the result differs from that of trim .
More than three parts are not possible with this indata, so.
gives the same result as the default, which is to be viewed as "an infinite number of parts".
Specifying 0 as the number of parts gives the same effect as option trim . If subexpressions are captured, empty subexpressions matched at the end are also stripped from the result if trim or is specified.
The trim behavior corresponds exactly to the Perl default. , where N is a positive integer, corresponds exactly to the Perl behavior with a positive numerical third parameter. The default behavior of split/3 corresponds to the Perl behavior when a negative integer is specified as the third parameter for the Perl routine.
Summary of options not previously described for function run/3 :
Specifies how the parts of the original string are presented in the result list. Valid types:
The variant of iodata() that gives the least copying of data with the current implementation (often a binary, but do not depend on it).
All parts returned as binaries.
All parts returned as lists of characters ("strings").
Groups together the part of the string with the parts of the string matching the subexpressions of the regular expression.
The return value from the function is in this case a list() of list() s. Each sublist begins with the string picked out of the subject string, followed by the parts matching each of the subexpressions in order of occurrence in the regular expression.
Specifies the number of parts the subject string is to be split into.
The number of parts is to be a positive integer for a specific maximum number of parts, and infinity for the maximum number of parts possible (the default). Specifying gives as many parts as possible disregarding empty parts at the end, the same as specifying trim .
Specifies that empty parts at the end of the result list are to be disregarded. The same as specifying . This corresponds to the default behavior of the split built-in function in Perl.
Perl-Like Regular Expression Syntax.
The following sections contain reference material for the regular expressions used by this module. The information is based on the PCRE documentation, with changes where this module behaves differently to the PCRE library.
PCRE Regular Expression Details.
The syntax and semantics of the regular expressions supported by PCRE are described in detail in the following sections. Perl's regular expressions are described in its own documentation, and regular expressions in general are covered in many books, some with copious examples. Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers regular expressions in great detail. This description of the PCRE regular expressions is intended as reference material.
The reference material is divided into the following sections:
Special Start-of-Pattern Items.
Some options that can be passed to compile/2 can also be set by special items at the start of a pattern. These are not Perl-compatible, but are provided to make these options accessible to pattern writers who are not able to change the program that processes the pattern. Any number of these items can appear, but they must all be together right at the start of the pattern string, and the letters must be in upper case.
Unicode support is basically UTF-8 based. To use Unicode characters, you either call compile/2 or run/3 with option unicode , or the pattern must start with one of these special sequences:
Both options give the same effect, the input string is interpreted as UTF-8. Notice that with these instructions, the automatic conversion of lists to UTF-8 is not performed by the re functions. Therefore, using these sequences is not recommended. Add option unicode when running compile/2 instead.
Some applications that allow their users to supply patterns can wish to restrict them to non-UTF data for security reasons. If option never_utf is set at compile time, (*UTF), and so on, are not allowed, and their appearance causes an error.
Unicode Property Support.
The following is another special sequence that can appear at the start of a pattern:
This has the same effect as setting option ucp : it causes sequences such as \d and \w to use Unicode properties to determine character types, instead of recognizing only characters with codes < 256 through a lookup table.
Disabling Startup Optimizations.
If a pattern starts with (*NO_START_OPT) , it has the same effect as setting option no_start_optimize at compile time.
PCRE supports five conventions for indicating line breaks in strings: a single CR (carriage return) character, a single LF (line feed) character, the two-character sequence CRLF, any of the three preceding, and any Unicode newline sequence.
A newline convention can also be specified by starting a pattern string with one of the following five sequences:
(*CR) Carriage return (*LF) Line feed (*CRLF) >Carriage return followed by line feed (*ANYCRLF) Any of the three above (*ANY) All Unicode newline sequences.
These override the default and the options specified to compile/2 . For example, the following pattern changes the convention to CR:
This pattern matches a\nb , as LF is no longer a newline. If more than one of them is present, the last one is used.
The newline convention affects where the circumflex and dollar assertions are true. It also affects the interpretation of the dot metacharacter when dotall is not set, and the behavior of \N. However, it does not affect what the \R escape sequence matches. By default, this is any Unicode newline sequence, for Perl compatibility. However, this can be changed; see the description of \R in section Newline Sequences . A change of the \R setting can be combined with a change of the newline convention.
Setting Match and Recursion Limits.
The caller of run/3 can set a limit on the number of times the internal match() function is called and on the maximum depth of recursive calls. These facilities are provided to catch runaway matches that are provoked by patterns with huge matching trees (a typical example is a pattern with nested unlimited repeats) and to avoid running out of system stack by too much recursion. When one of these limits is reached, pcre_exec() gives an error return. The limits can also be set by items at the start of the pattern of the following forms:
Here d is any number of decimal digits. However, the value of the setting must be less than the value set by the caller of run/3 for it to have any effect. That is, the pattern writer can lower the limit set by the programmer, but not raise it. If there is more than one setting of one of these limits, the lower value is used.
The default value for both the limits is 10,000,000 in the Erlang VM. Notice that the recursion limit does not affect the stack depth of the VM, as PCRE for Erlang is compiled in such a way that the match function never does recursion on the C stack.
Note that LIMIT_MATCH and LIMIT_RECURSION can only reduce the value of the limits set by the caller, not increase them.
Characters and Metacharacters.
A regular expression is a pattern that is matched against a subject string from left to right. Most characters stand for themselves in a pattern and match the corresponding characters in the subject. As a trivial example, the following pattern matches a portion of a subject string that is identical to itself:
When caseless matching is specified (option caseless ), letters are matched independently of case.
The power of regular expressions comes from the ability to include alternatives and repetitions in the pattern. These are encoded in the pattern by the use of metacharacters , which do not stand for themselves but instead are interpreted in some special way.
Two sets of metacharacters exist: those that are recognized anywhere in the pattern except within square brackets, and those that are recognized within square brackets. Outside square brackets, the metacharacters are as follows:
\ General escape character with many uses ^ Assert start of string (or line, in multiline mode) $ Assert end of string (or line, in multiline mode) . Match any character except newline (by default) [ Start character class definition | Start of alternative branch ( Start subpattern ) End subpattern ? Extends the meaning of (, also 0 or 1 quantifier, also quantifier minimizer * 0 or more quantifiers + 1 or more quantifier, also "possessive quantifier" Start min/max quantifier.
Part of a pattern within square brackets is called a "character class". The following are the only metacharacters in a character class:
\ General escape character ^ Negate the class, but only if the first character - Indicates character range [ Posix character class (only if followed by Posix syntax) ] Terminates the character class.
The following sections describe the use of each metacharacter.
The backslash character has many uses. First, if it is followed by a character that is not a number or a letter, it takes away any special meaning that a character can have. This use of backslash as an escape character applies both inside and outside character classes.
For example, if you want to match a * character, you write \* in the pattern. This escaping action applies if the following character would otherwise be interpreted as a metacharacter, so it is always safe to precede a non-alphanumeric with backslash to specify that it stands for itself. In particular, if you want to match a backslash, write \\.
In unicode mode, only ASCII numbers and letters have any special meaning after a backslash. All other characters (in particular, those whose code points are > 127) are treated as literals.
If a pattern is compiled with option extended , whitespace in the pattern (other than in a character class) and characters between a # outside a character class and the next newline are ignored. An escaping backslash can be used to include a whitespace or # character as part of the pattern.
To remove the special meaning from a sequence of characters, put them between \Q and \E. This is different from Perl in that $ and are handled as literals in \Q. \E sequences in PCRE, while $ and cause variable interpolation in Perl. Notice the following examples:
The \Q. \E sequence is recognized both inside and outside character classes. An isolated \E that is not preceded by \Q is ignored. If \Q is not followed by \E later in the pattern, the literal interpretation continues to the end of the pattern (that is, \E is assumed at the end). If the isolated \Q is inside a character class, this causes an error, as the character class is not terminated.
A second use of backslash provides a way of encoding non-printing characters in patterns in a visible manner. There is no restriction on the appearance of non-printing characters, apart from the binary zero that terminates a pattern. When a pattern is prepared by text editing, it is often easier to use one of the following escape sequences than the binary character it represents:
\a Alarm, that is, the BEL character (hex 07) \cx "Control-x", where x is any ASCII character \e Escape (hex 1B) \f Form feed (hex 0C) \n Line feed (hex 0A) \r Carriage return (hex 0D) \t Tab (hex 09) \0dd Character with octal code 0dd \ddd Character with octal code ddd, or back reference \o character with octal code ddd.. \xhh Character with hex code hh \x Character with hex code hhh..
Note that \0dd is always an octal code, and that \8 and \9 are the literal characters "8" and "9".
The \c facility was designed for use with ASCII characters, but with the extension to Unicode it is even less useful than it once was.
After \0 up to two further octal digits are read. If there are fewer than two digits, just those that are present are used. Thus the sequence \0\x\015 specifies two binary zeros followed by a CR character (code value 13). Make sure you supply two digits after the initial zero if the pattern character that follows is itself an octal digit.
The escape \o must be followed by a sequence of octal digits, enclosed in braces. An error occurs if this is not the case. This escape is a recent addition to Perl; it provides way of specifying character code points as octal numbers greater than 0777, and it also allows octal numbers and back references to be unambiguously specified.
For greater clarity and unambiguity, it is best to avoid following \ by a digit greater than zero. Instead, use \o<> or \x<> to specify character numbers, and \g<> to specify back references. The following paragraphs describe the old, ambiguous syntax.
The handling of a backslash followed by a digit other than 0 is complicated, and Perl has changed in recent releases, causing PCRE also to change. Outside a character class, PCRE reads the digit and any following digits as a decimal number. If the number is < 8, or if there have been at least that many previous capturing left parentheses in the expression, the entire sequence is taken as a back reference . A description of how this works is provided later, following the discussion of parenthesized subpatterns.
Inside a character class, or if the decimal number following \ is > 7 and there have not been that many capturing subpatterns, PCRE handles \8 and \9 as the literal characters "8" and "9", and otherwise re-reads up to three octal digits following the backslash, and using them to generate a data character. Any subsequent digits stand for themselves. Por exemplo:
\040 Another way of writing an ASCII space \40 The same, provided there are < 40 previous capturing subpatterns \7 Always a back reference \11 Can be a back reference, or another way of writing a tab \011 Always a tab \0113 A tab followed by character "3" \113 Can be a back reference, otherwise the character with octal code 113 \377 Can be a back reference, otherwise value 255 (decimal) \81 Either a back reference, or the two characters "8" and "1"
Notice that octal values >= 100 that are specified using this syntax must not be introduced by a leading zero, as no more than three octal digits are ever read.
Characters whose value is less than 256 can be defined by either of the two syntaxes for \x. There is no difference in the way they are handled. For example, \xdc is exactly the same as \x .
Constraints on character values.
Characters that are specified using octal or hexadecimal numbers are limited to certain values, as follows:
& lt; 0x10ffff and a valid codepoint.
Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called "surrogate" codepoints), and 0xffef.
Escape sequences in character classes.
All the sequences that define a single character value can be used both inside and outside character classes. Also, inside a character class, \b is interpreted as the backspace character (hex 08).
\N is not allowed in a character class. \B, \R, and \X are not special inside a character class. Like other unrecognized escape sequences, they are treated as the literal characters "B", "R", and "X". Outside a character class, these sequences have different meanings.
Unsupported Escape Sequences.
In Perl, the sequences \l, \L, \u, and \U are recognized by its string handler and used to modify the case of following characters. PCRE does not support these escape sequences.
Absolute and Relative Back References.
The sequence \g followed by an unsigned or a negative number, optionally enclosed in braces, is an absolute or relative back reference. A named back reference can be coded as \g . Back references are discussed later, following the discussion of parenthesized subpatterns.
Absolute and Relative Subroutine Calls.
For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is alternative syntax for referencing a subpattern as a "subroutine". Details are discussed later. Notice that \g (Perl syntax) and \g<. & gt; (Oniguruma syntax) are not synonymous. The former is a back reference and the latter is a subroutine call.
Another use of backslash is for specifying generic character types:
\d Any decimal digit \D Any character that is not a decimal digit \h Any horizontal whitespace character \H Any character that is not a horizontal whitespace character \s Any whitespace character \S Any character that is not a whitespace character \v Any vertical whitespace character \V Any character that is not a vertical whitespace character \w Any "word" character \W Any "non-word" character.
There is also the single sequence \N, which matches a non-newline character. This is the same as the "." metacharacter when dotall is not set. Perl also uses \N to match characters by name, but PCRE does not support this.
Each pair of lowercase and uppercase escape sequences partitions the complete set of characters into two disjoint sets. Any given character matches one, and only one, of each pair. The sequences can appear both inside and outside character classes. They each match one character of the appropriate type. If the current matching point is at the end of the subject string, all fail, as there is no character to match.
For compatibility with Perl, \s did not used to match the VT character (code 11), which made it different from the the POSIX "space" class. However, Perl added VT at release 5.18, and PCRE followed suit at release 8.34. The default \s characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space (32), which are defined as white space in the "C" locale. This list may vary if locale-specific matching is taking place. For example, in some locales the "non-breaking space" character (\xA0) is recognized as white space, and in others the VT character is not.
A "word" character is an underscore or any character that is a letter or a digit. By default, the definition of letters and digits is controlled by the PCRE low-valued character tables, in Erlang's case (and without option unicode ), the ISO Latin-1 character set.
By default, in unicode mode, characters with values > 255, that is, all characters outside the ISO Latin-1 character set, never match \d, \s, or \w, and always match \D, \S, and \W. These sequences retain their original meanings from before UTF support was available, mainly for efficiency reasons. However, if option ucp is set, the behavior is changed so that Unicode properties are used to determine character types, as follows:
\d Any character that \p matches (decimal digit) \s Any character that \p or \h or \v \w Any character that matches \p or \p matches, plus underscore.
The uppercase escapes match the inverse sets of characters. Notice that \d matches only decimal digits, while \w matches any Unicode digit, any Unicode letter, and underscore. Notice also that ucp affects \b and \B, as they are defined in terms of \w and \W. Matching these sequences is noticeably slower when ucp is set.
The sequences \h, \H, \v, and \V are features that were added to Perl in release 5.10. In contrast to the other sequences, which match only ASCII characters by default, these always match certain high-valued code points, regardless if ucp is set.
The following are the horizontal space characters:
U+0009 Horizontal tab (HT) U+0020 Space U+00A0 Non-break space U+1680 Ogham space mark U+180E Mongolian vowel separator U+2000 En quad U+2001 Em quad U+2002 En space U+2003 Em space U+2004 Three-per-em space U+2005 Four-per-em space U+2006 Six-per-em space U+2007 Figure space U+2008 Punctuation space U+2009 Thin space U+200A Hair space U+202F Narrow no-break space U+205F Medium mathematical space U+3000 Ideographic space.
The following are the vertical space characters:
U+000A Line feed (LF) U+000B Vertical tab (VT) U+000C Form feed (FF) U+000D Carriage return (CR) U+0085 Next line (NEL) U+2028 Line separator U+2029 Paragraph separator.
In 8-bit, non-UTF-8 mode, only the characters with code points < 256 are relevant.
Outside a character class, by default, the escape sequence \R matches any Unicode newline sequence. In non-UTF-8 mode, \R is equivalent to the following:
This is an example of an "atomic group", details are provided below.
This particular group matches either the two-character sequence CR followed by LF, or one of the single characters LF (line feed, U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next line, U+0085). The two-character sequence is treated as a single unit that cannot be split.
In Unicode mode, two more characters whose code points are > 255 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029). Unicode character property support is not needed for these characters to be recognized.
\R can be restricted to match only CR, LF, or CRLF (instead of the complete set of Unicode line endings) by setting option bsr_anycrlf either at compile time or when the pattern is matched. (BSR is an acronym for "backslash R".) This can be made the default when PCRE is built; if so, the other behavior can be requested through option bsr_unicode . These settings can also be specified by starting a pattern string with one of the following sequences:
(*BSR_ANYCRLF) CR, LF, or CRLF only (*BSR_UNICODE) Any Unicode newline sequence.
These override the default and the options specified to the compiling function, but they can themselves be overridden by options specified to a matching function. Notice that these special settings, which are not Perl-compatible, are recognized only at the very start of a pattern, and that they must be in upper case. If more than one of them is present, the last one is used. They can be combined with a change of newline convention; for example, a pattern can start with:
They can also be combined with the (*UTF8), (*UTF), or (*UCP) special sequences. Inside a character class, \R is treated as an unrecognized escape sequence, and so matches the letter "R" by default.
Unicode Character Properties.
Three more escape sequences that match characters with specific properties are available. When in 8-bit non-UTF-8 mode, these sequences are limited to testing characters whose code points are < 256, but they do work in this mode. The following are the extra escape sequences:
The property names represented by xx above are limited to the Unicode script names, the general category properties, "Any", which matches any character (including newline), and some special PCRE properties (described in the next section). Other Perl properties, such as "InMusicalSymbols", are currently not supported by PCRE. Notice that \P does not match any characters and always causes a match failure.
Sets of Unicode characters are defined as belonging to certain scripts. A character from one of these sets can be matched using a script name, for example:
Those that are not part of an identified script are lumped together as "Common". The following is the current list of scripts:
Each character has exactly one Unicode general category property, specified by a two-letter acronym. For compatibility with Perl, negation can be specified by including a circumflex between the opening brace and the property name. For example, \p is the same as \P .
If only one letter is specified with \p or \P, it includes all the general category properties that start with that letter. In this case, in the absence of negation, the curly brackets in the escape sequence are optional. The following two examples have the same effect:
The following general category property codes are supported:
The special property L& is also supported. It matches a character that has the Lu, Ll, or Lt property, that is, a letter that is not classified as a modifier or "other".
The Cs (Surrogate) property applies only to characters in the range U+D800 to U+DFFF. Such characters are invalid in Unicode strings and so cannot be tested by PCRE. Perl does not support the Cs property.
The long synonyms for property names supported by Perl (such as \p ) are not supported by PCRE. It is not permitted to prefix any of these properties with "Is".
No character in the Unicode table has the Cn (unassigned) property. This property is instead assumed for any code point that is not in the Unicode table.
Specifying caseless matching does not affect these escape sequences. For example, \p always matches only uppercase letters. This is different from the behavior of current versions of Perl.
Matching characters by Unicode property is not fast, as PCRE must do a multistage table lookup to find a character property. That is why the traditional escape sequences such as \d and \w do not use Unicode properties in PCRE by default. However, you can make them do so by setting option ucp or by starting the pattern with (*UCP).
Extended Grapheme Clusters.
The \X escape matches any number of Unicode characters that form an "extended grapheme cluster", and treats the sequence as an atomic group (see below). Up to and including release 8.31, PCRE matched an earlier, simpler definition that was equivalent to (?>\PM\pM*) . That is, it matched a character without the "mark" property, followed by zero or more characters with the "mark" property. Characters with the "mark" property are typically non-spacing accents that affect the preceding character.
This simple definition was extended in Unicode to include more complicated kinds of composite character by giving each character a grapheme breaking property, and creating rules that use these properties to define the boundaries of extended grapheme clusters. In PCRE releases later than 8.31, \X matches one of these clusters.
\X always matches at least one character. Then it decides whether to add more characters according to the following rules for ending a cluster:
End at the end of the subject string.
Do not end between CR and LF; otherwise end after any control character.
Do not break Hangul (a Korean script) syllable sequences. Hangul characters are of five types: L, V, T, LV, and LVT. An L character can be followed by an L, V, LV, or LVT character. An LV or V character can be followed by a V or T character. An LVT or T character can be followed only by a T character.
Do not end before extending characters or spacing marks. Characters with the "mark" property always have the "extend" grapheme breaking property.
Do not end after prepend characters.
Otherwise, end the cluster.
PCRE Additional Properties.
In addition to the standard Unicode properties described earlier, PCRE supports four more that make it possible to convert traditional escape sequences, such as \w and \s to use Unicode properties. PCRE uses these non-standard, non-Perl properties internally when the ucp option is passed. However, they can also be used explicitly. The properties are as follows:
Any alphanumeric character. Matches characters that have either the L (letter) or the N (number) property.
Any Posix space character. Matches the characters tab, line feed, vertical tab, form feed, carriage return, and any other character that has the Z (separator) property.
Any Perl space character. Matches the same as Xps, except that vertical tab is excluded.
Any Perl "word" character. Matches the same characters as Xan, plus underscore.
Perl and POSIX space are now the same. Perl added VT to its space character set at release 5.18 and PCRE changed at release 8.34.
Xan matches characters that have either the L (letter) or the N (number) property. Xps matches the characters tab, linefeed, vertical tab, form feed, or carriage return, and any other character that has the Z (separator) property. Xsp is the same as Xps; it used to exclude vertical tab, for Perl compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd matches the same characters as Xan, plus underscore.
There is another non-standard property, Xuc, which matches any character that can be represented by a Universal Character Name in C++ and other programming languages. These are the characters $, , ` (grave accent), and all characters with Unicode code points >= U+00A0, except for the surrogates U+D800 to U+DFFF. Notice that most base (ASCII) characters are excluded. (Universal Character Names are of the form \uHHHH or \UHHHHHHHH, where H is a hexadecimal digit. Notice that the Xuc property does not match these sequences but the characters that they represent.)
Resetting the Match Start.
The escape sequence \K causes any previously matched characters not to be included in the final matched sequence. For example, the following pattern matches "foobar", but reports that it has matched "bar":
This feature is similar to a lookbehind assertion (described below). However, in this case, the part of the subject before the real match does not have to be of fixed length, as lookbehind assertions do. The use of \K does not interfere with the setting of captured substrings. For example, when the following pattern matches "foobar", the first substring is still set to "foo":
Perl documents that the use of \K within assertions is "not well defined". In PCRE, \K is acted upon when it occurs inside positive assertions, but is ignored in negative assertions. Note that when a pattern such as (?=ab\K) matches, the reported start of the match can be greater than the end of the match.
The final use of backslash is for certain simple assertions. An assertion specifies a condition that must be met at a particular point in a match, without consuming any characters from the subject string. The use of subpatterns for more complicated assertions is described below. The following are the backslashed assertions:
\b Matches at a word boundary. \B Matches when not at a word boundary. \A Matches at the start of the subject. \Z Matches at the end of the subject, and before a newline at the end of the subject. \z Matches only at the end of the subject. \G Matches at the first matching position in the subject.
Inside a character class, \b has a different meaning; it matches the backspace character. If any other of these assertions appears in a character class, by default it matches the corresponding literal character (for example, \B matches the letter B).
A word boundary is a position in the subject string where the current character and the previous character do not both match \w or \W (that is, one matches \w and the other matches \W), or the start or end of the string if the first or last character matches \w, respectively. In UTF mode, the meanings of \w and \W can be changed by setting option ucp . When this is done, it also affects \b and \B. PCRE and Perl do not have a separate "start of word" or "end of word" metasequence. However, whatever follows \b normally determines which it is. For example, the fragment \ba matches "a" at the start of a word.
The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described in the next section) in that they only ever match at the very start and end of the subject string, whatever options are set. Thus, they are independent of multiline mode. These three assertions are not affected by options notbol or noteol , which affect only the behavior of the circumflex and dollar metacharacters. However, if argument startoffset of run/3 is non-zero, indicating that matching is to start at a point other than the beginning of the subject, \A can never match. The difference between \Z and \z is that \Z matches before a newline at the end of the string and at the very end, while \z matches only at the end.
The \G assertion is true only when the current matching position is at the start point of the match, as specified by argument startoffset of run/3 . It differs from \A when the value of startoffset is non-zero. By calling run/3 multiple times with appropriate arguments, you can mimic the Perl option /g , and it is in this kind of implementation where \G can be useful.
Notice, however, that the PCRE interpretation of \G, as the start of the current match, is subtly different from Perl, which defines it as the end of the previous match. In Perl, these can be different when the previously matched string was empty. As PCRE does only one match at a time, it cannot reproduce this behavior.
If all the alternatives of a pattern begin with \G, the expression is anchored to the starting match position, and the "anchored" flag is set in the compiled regular expression.
Circumflex and Dollar.
The circumflex and dollar metacharacters are zero-width assertions. That is, they test for a particular condition to be true without consuming any characters from the subject string.
Outside a character class, in the default matching mode, the circumflex character is an assertion that is true only if the current matching point is at the start of the subject string. If argument startoffset of run/3 is non-zero, circumflex can never match if option multiline is unset. Inside a character class, circumflex has an entirely different meaning (see below).
Circumflex needs not to be the first character of the pattern if some alternatives are involved, but it is to be the first thing in each alternative in which it appears if the pattern is ever to match that branch. If all possible alternatives start with a circumflex, that is, if the pattern is constrained to match only at the start of the subject, it is said to be an "anchored" pattern. (There are also other constructs that can cause a pattern to be anchored.)
The dollar character is an assertion that is true only if the current matching point is at the end of the subject string, or immediately before a newline at the end of the string (by default). Notice however that it does not match the newline. Dollar needs not to be the last character of the pattern if some alternatives are involved, but it is to be the last item in any branch in which it appears. Dollar has no special meaning in a character class.
The meaning of dollar can be changed so that it matches only at the very end of the string, by setting option dollar_endonly at compile time. This does not affect the \Z assertion.
The meanings of the circumflex and dollar characters are changed if option multiline is set. When this is the case, a circumflex matches immediately after internal newlines and at the start of the subject string. It does not match after a newline that ends the string. A dollar matches before any newlines in the string, and at the very end, when multiline is set. When newline is specified as the two-character sequence CRLF, isolated CR and LF characters do not indicate newlines.
For example, the pattern /^abc$/ matches the subject string "def\nabc" (where \n represents a newline) in multiline mode, but not otherwise. So, patterns that are anchored in single-line mode because all branches start with ^ are not anchored in multiline mode, and a match for circumflex is possible when argument startoffset of run/3 is non-zero. Option dollar_endonly is ignored if multiline is set.
Notice that the sequences \A, \Z, and \z can be used to match the start and end of the subject in both modes. If all branches of a pattern start with \A, it is always anchored, regardless if multiline is set.
Full Stop (Period, Dot) and \N.
Outside a character class, a dot in the pattern matches any character in the subject string except (by default) a character that signifies the end of a line.
When a line ending is defined as a single character, dot never matches that character. When the two-character sequence CRLF is used, dot does not match CR if it is immediately followed by LF, otherwise it matches all characters (including isolated CRs and LFs). When any Unicode line endings are recognized, dot does not match CR, LF, or any of the other line-ending characters.
The behavior of dot regarding newlines can be changed. If option dotall is set, a dot matches any character, without exception. If the two-character sequence CRLF is present in the subject string, it takes two dots to match it.
The handling of dot is entirely independent of the handling of circumflex and dollar, the only relationship is that both involve newlines. Dot has no special meaning in a character class.
The escape sequence \N behaves like a dot, except that it is not affected by option PCRE_DOTALL . That is, it matches any character except one that signifies the end of a line. Perl also uses \N to match characters by name but PCRE does not support this.
Matching a Single Data Unit.
Outside a character class, the escape sequence \C matches any data unit, regardless if a UTF mode is set. One data unit is one byte. Unlike a dot, \C always matches line-ending characters. The feature is provided in Perl to match individual bytes in UTF-8 mode, but it is unclear how it can usefully be used. As \C breaks up characters into individual data units, matching one unit with \C in a UTF mode means that the remaining string can start with a malformed UTF character. This has undefined results, as PCRE assumes that it deals with valid UTF strings.
PCRE does not allow \C to appear in lookbehind assertions (described below) in a UTF mode, as this would make it impossible to calculate the length of the lookbehind.
The \C escape sequence is best avoided. However, one way of using it that avoids the problem of malformed UTF characters is to use a lookahead to check the length of the next character, as in the following pattern, which can be used with a UTF-8 string (ignore whitespace and line breaks):
A group that starts with (?| resets the capturing parentheses numbers in each alternative (see section Duplicate Subpattern Numbers ). The assertions at the start of each branch check the next UTF-8 character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The individual bytes of the character are then captured by the appropriate number of groups.
Square Brackets and Character Classes.
An opening square bracket introduces a character class, terminated by a closing square bracket. A closing square bracket on its own is not special by default. However, if option PCRE_JAVASCRIPT_COMPAT is set, a lone closing square bracket causes a compile-time error. If a closing square bracket is required as a member of the class, it is to be the first data character in the class (after an initial circumflex, if present) or escaped with a backslash.
A character class matches a single character in the subject. In a UTF mode, the character can be more than one data unit long. A matched character must be in the set of characters defined by the class, unless the first character in the class definition is a circumflex, in which case the subject character must not be in the set defined by the class. If a circumflex is required as a member of the class, ensure that it is not the first character, or escape it with a backslash.
For example, the character class [aeiou] matches any lowercase vowel, while [^aeiou] matches any character that is not a lowercase vowel. Notice that a circumflex is just a convenient notation for specifying the characters that are in the class by enumerating those that are not. A class that starts with a circumflex is not an assertion; it still consumes a character from the subject string, and therefore it fails if the current pointer is at the end of the string.
In UTF-8 mode, characters with values > 255 (0xffff) can be included in a class as a literal string of data units, or by using the \x.
When caseless matching is set, any letters in a class represent both their uppercase and lowercase versions. For example, a caseless [aeiou] matches "A" and "a", and a caseless [^aeiou] does not match "A", but a caseful version would. In a UTF mode, PCRE always understands the concept of case for characters whose values are < 256, so caseless matching is always possible. For characters with higher values, the concept of case is supported only if PCRE is compiled with Unicode property support. If you want to use caseless matching in a UTF mode for characters >=, ensure that PCRE is compiled with Unicode property support and with UTF support.
Characters that can indicate line breaks are never treated in any special way when matching character classes, whatever line-ending sequence is in use, and whatever setting of options PCRE_DOTALL and PCRE_MULTILINE is used. A class such as [^a] always matches one of these characters.
The minus (hyphen) character can be used to specify a range of characters in a character class. For example, [d-m] matches any letter between d and m, inclusive. If a minus character is required in a class, it must be escaped with a backslash or appear in a position where it cannot be interpreted as indicating a range, typically as the first or last character in the class, or immediately after a range. For example, [b-d-z] matches letters in the range b to d, a hyphen character, or z.
The literal character "]" cannot be the end character of a range. A pattern such as [W-]46] is interpreted as a class of two characters ("W" and "-") followed by a literal string "46]", so it would match "W46]" or "-46]". However, if "]" is escaped with a backslash, it is interpreted as the end of range, so [W-\]46] is interpreted as a class containing a range followed by two other characters. The octal or hexadecimal representation of "]" can also be used to end a range.
An error is generated if a POSIX character class (see below) or an escape sequence other than one that defines a single character appears at a point where a range ending character is expected. For example, [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not.
Ranges operate in the collating sequence of character values. They can also be used for characters specified numerically, for example, [\000-\037]. Ranges can include any characters that are valid for the current mode.
If a range that includes letters is used when caseless matching is set, it matches the letters in either case. For example, [W-c] is equivalent to [][\\^_`wxyzabc], matched caselessly. In a non-UTF mode, if character tables for a French locale are in use, [\xc8-\xcb] matches accented E characters in both cases. In UTF modes, PCRE supports the concept of case for characters with values > 255 only when it is compiled with Unicode property support.
The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, \w, and \W can appear in a character class, and add the characters that they match to the class. For example, [\dABCDEF] matches any hexadecimal digit. In UTF modes, option ucp affects the meanings of \d, \s, \w and their uppercase partners, just as it does when they appear outside a character class, as described in section Generic Character Types earlier. The escape sequence \b has a different meaning inside a character class; it matches the backspace character. The sequences \B, \N, \R, and \X are not special inside a character class. Like any other unrecognized escape sequences, they are treated as the literal characters "B", "N", "R", and "X".
A circumflex can conveniently be used with the uppercase character types to specify a more restricted set of characters than the matching lowercase type. For example, class [^\W_] matches any letter or digit, but not underscore, while [\w] includes underscore. A positive character class is to be read as "something OR something OR . " and a negative class as "NOT something AND NOT something AND NOT . ".
Only the following metacharacters are recognized in character classes:
Backslash Hyphen (only where it can be interpreted as specifying a range) Circumflex (only at the start) Opening square bracket (only when it can be interpreted as introducing a Posix class name, or for a special compatibility feature; see the next two sections) Terminating closing square bracket.
However, escaping other non-alphanumeric characters does no harm.
Posix Character Classes.
Perl supports the Posix notation for character classes. This uses names enclosed by [: and :] within the enclosing square brackets. PCRE also supports this notation. For example, the following matches "0", "1", any alphabetic character, or "%":
The following are the supported class names:
alnum Letters and digits alpha Letters ascii Character codes 0-127 blank Space or tab only cntrl Control characters digit Decimal digits (same as \d) graph Printing characters, excluding space lower Lowercase letters print Printing characters, including space punct Printing characters, excluding letters, digits, and space space Whitespace (the same as \s from PCRE 8.34) upper Uppercase letters word "Word" characters (same as \w) xdigit Hexadecimal digits.
The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and space (32). If locale-specific matching is taking place, the list of space characters may be different; there may be fewer or more of them. "Space" used to be different to \s, which did not include VT, for Perl compatibility. However, Perl changed at release 5.18, and PCRE followed at release 8.34. "Space" and \s now match the same set of characters.
The name "word" is a Perl extension, and "blank" is a GNU extension from Perl 5.8. Another Perl extension is negation, which is indicated by a ^ character after the colon. For example, the following matches "1", "2", or any non-digit:
PCRE (and Perl) also recognize the Posix syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not supported, and an error is given if they are encountered.
By default, characters with values > 255 do not match any of the Posix character classes. However, if option PCRE_UCP is passed to pcre_compile() , some of the classes are changed so that Unicode character properties are used. This is achieved by replacing certain Posix classes by other sequences, as follows:
Negated versions, such as [:^alpha:], use \P instead of \p. Three other POSIX classes are handled specially in UCP mode:
This matches characters that have glyphs that mark the page when printed. In Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf properties, except for:
Arabic Letter Mark.
Mongolian Vowel Separator.
This matches the same characters as [:graph:] plus space characters that are not controls, that is, characters with the Zs property.
This matches all characters that have the Unicode P (punctuation) property, plus those characters whose code points are less than 128 that have the S (Symbol) property.
The other POSIX classes are unchanged, and match only characters with code points less than 128.
Compatibility Feature for Word Boundaries.
In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of word". PCRE treats these items as follows:
is converted to \b(?=\w)
is converted to \b(?<=\w)
Only these exact character sequences are recognized. A sequence such as [a[:<:]b] provokes error for an unrecognized POSIX class name. This support is not compatible with Perl. It is provided to help migrations from other environments, and is best not used in any new patterns. Note that \b matches at the start and the end of a word (see "Simple assertions" above), and in a Perl-style pattern the preceding or following character normally shows which is wanted, without the need for the assertions that are used above in order to give exactly the POSIX behaviour.
Vertical Bar.
Vertical bar characters are used to separate alternative patterns. For example, the following pattern matches either "gilbert" or "sullivan":
Any number of alternatives can appear, and an empty alternative is permitted (matching the empty string). The matching process tries each alternative in turn, from left to right, and the first that succeeds is used. If the alternatives are within a subpattern (defined in section Subpatterns ), "succeeds" means matching the remaining main pattern and the alternative in the subpattern.
Internal Option Setting.
The settings of the Perl-compatible options caseless , multiline , dotall , and extended can be changed from within the pattern by a sequence of Perl option letters enclosed between "(?" and ")". The option letters are as follows:
For example, (?im) sets caseless, multiline matching. These options can also be unset by preceding the letter with a hyphen. A combined setting and unsetting such as (?im-sx) , which sets caseless and multiline , while unsetting dotall and extended , is also permitted. If a letter appears both before and after the hyphen, the option is unset.
The PCRE-specific options dupnames , ungreedy , and extra can be changed in the same way as the Perl-compatible options by using the characters J, U, and X respectively.
When one of these option changes occurs at top-level (that is, not inside subpattern parentheses), the change applies to the remainder of the pattern that follows.
An option change within a subpattern (see section Subpatterns ) affects only that part of the subpattern that follows it. So, the following matches abc and aBc and no other strings (assuming caseless is not used):
By this means, options can be made to have different settings in different parts of the pattern. Any changes made in one alternative do carry on into subsequent branches within the same subpattern. Por exemplo:
matches "ab", "aB", "c", and "C", although when matching "C" the first branch is abandoned before the option setting. This is because the effects of option settings occur at compile time. There would be some weird behavior otherwise.
Other PCRE-specific options can be set by the application when the compiling or matching functions are called. Sometimes the pattern can contain special leading sequences, such as (*CRLF), to override what the application has set or what has been defaulted. Details are provided in section Newline Sequences earlier.
The (*UTF8) and (*UCP) leading sequences can be used to set UTF and Unicode property modes. They are equivalent to setting options unicode and ucp , respectively. The (*UTF) sequence is a generic version that can be used with any of the libraries. However, the application can set option never_utf , which locks out the use of the (*UTF) sequences.
Subpatterns.
Subpatterns are delimited by parentheses (round brackets), which can be nested. Turning part of a pattern into a subpattern does two things:
It localizes a set of alternatives. For example, the following pattern matches "cataract", "caterpillar", or "cat":
Without the parentheses, it would match "cataract", "erpillar", or an empty string.
It sets up the subpattern as a capturing subpattern. That is, when the complete pattern matches, that portion of the subject string that matched the subpattern is passed back to the caller through the return value of run/3 .
Opening parentheses are counted from left to right (starting from 1) to obtain numbers for the capturing subpatterns. For example, if the string "the red king" is matched against the following pattern, the captured substrings are "red king", "red", and "king", and are numbered 1, 2, and 3, respectively:
It is not always helpful that plain parentheses fulfill two functions. Often a grouping subpattern is required without a capturing requirement. If an opening parenthesis is followed by a question mark and a colon, the subpattern does not do any capturing, and is not counted when computing the number of any subsequent capturing subpatterns. For example, if the string "the white queen" is matched against the following pattern, the captured substrings are "white queen" and "queen", and are numbered 1 and 2:
The maximum number of capturing subpatterns is 65535.
As a convenient shorthand, if any option settings are required at the start of a non-capturing subpattern, the option letters can appear between "?" and ":". Thus, the following two patterns match the same set of strings:
As alternative branches are tried from left to right, and options are not reset until the end of the subpattern is reached, an option setting in one branch does affect subsequent branches, so the above patterns match both "SUNDAY" and "Saturday".
Duplicate Subpattern Numbers.
Perl 5.10 introduced a feature where each alternative in a subpattern uses the same numbers for its capturing parentheses. Such a subpattern starts with (?| and is itself a non-capturing subpattern. For example, consider the following pattern:
As the two alternatives are inside a (?| group, both sets of capturing parentheses are numbered one. Thus, when the pattern matches, you can look at captured substring number one, whichever alternative matched. This construct is useful when you want to capture a part, but not all, of one of many alternatives. Inside a (?| group, parentheses are numbered as usual, but the number is reset at the start of each branch. The numbers of any capturing parentheses that follow the subpattern start after the highest number used in any branch. The following example is from the Perl documentation; the numbers underneath show in which buffer the captured content is stored:
A back reference to a numbered subpattern uses the most recent value that is set for that number by any subpattern. The following pattern matches "abcabc" or "defdef":
In contrast, a subroutine call to a numbered subpattern always refers to the first one in the pattern with the given number. The following pattern matches "abcabc" or "defabc":
If a condition test for a subpattern having matched refers to a non-unique number, the test is true if any of the subpatterns of that number have matched.
An alternative approach using this "branch reset" feature is to use duplicate named subpatterns, as described in the next section.
Named Subpatterns.
Identifying capturing parentheses by number is simple, but it can be hard to keep track of the numbers in complicated regular expressions. Also, if an expression is modified, the numbers can change. To help with this difficulty, PCRE supports the naming of subpatterns. This feature was not added to Perl until release 5.10. Python had the feature earlier, and PCRE introduced it at release 4.0, using the Python syntax. PCRE now supports both the Perl and the Python syntax. Perl allows identically numbered subpatterns to have different names, but PCRE does not.
In PCRE, a subpattern can be named in one of three ways: (?<name>. ) or (?'name'. ) as in Perl, or (?P<name>. ) as in Python. References to capturing parentheses from other parts of the pattern, such as back references, recursion, and conditions, can be made by name and by number.
Names consist of up to 32 alphanumeric characters and underscores, but must start with a non-digit. Named capturing parentheses are still allocated numbers as well as names, exactly as if the names were not present. The capture specification to run/3 can use named values if they are present in the regular expression.
By default, a name must be unique within a pattern, but this constraint can be relaxed by setting option dupnames at compile time. (Duplicate names are also always permitted for subpatterns with the same number, set up as described in the previous section.) Duplicate names can be useful for patterns where only one instance of the named parentheses can match. Suppose that you want to match the name of a weekday, either as a 3-letter abbreviation or as the full name, and in both cases you want to extract the abbreviation. The following pattern (ignoring the line breaks) does the job:
There are five capturing substrings, but only one is ever set after a match. (An alternative way of solving this problem is to use a "branch reset" subpattern, as described in the previous section.)
For capturing named subpatterns which names are not unique, the first matching occurrence (counted from left to right in the subject) is returned from run/3 , if the name is specified in the values part of the capture statement. The all_names capturing value matches all the names in the same way.
You cannot use different names to distinguish between two subpatterns with the same number, as PCRE uses only the numbers when matching. For this reason, an error is given at compile time if different names are specified to subpatterns with the same number. However, you can specify the same name to subpatterns with the same number, even when dupnames is not set.
Repetição.
Repetition is specified by quantifiers, which can follow any of the following items:
A literal data character The dot metacharacter The \C escape sequence The \X escape sequence The \R escape sequence An escape such as \d or \pL that matches a single character A character class A back reference (see the next section) A parenthesized subpattern (including assertions) A subroutine call to a subpattern (recursive or otherwise)
The general repetition quantifier specifies a minimum and maximum number of permitted matches, by giving the two numbers in curly brackets (braces), separated by a comma. The numbers must be < 65536, and the first must be less than or equal to the second. For example, the following matches "zz", "zzz", or "zzzz":
A closing brace on its own is not a special character. If the second number is omitted, but the comma is present, there is no upper limit. If the second number and the comma are both omitted, the quantifier specifies an exact number of required matches. Thus, the following matches at least three successive vowels, but can match many more:
The following matches exactly eight digits:
An opening curly bracket that appears in a position where a quantifier is not allowed, or one that does not match the syntax of a quantifier, is taken as a literal character. For example, is not a quantifier, but a literal string of four characters.
In Unicode mode, quantifiers apply to characters rather than to individual data units. Thus, for example, \x matches two characters, each of which is represented by a 2-byte sequence in a UTF-8 string. Similarly, \X matches three Unicode extended grapheme clusters, each of which can be many data units long (and they can be of different lengths).
The quantifier is permitted, causing the expression to behave as if the previous item and the quantifier were not present. This can be useful for subpatterns that are referenced as subroutines from elsewhere in the pattern (but see also section Defining Subpatterns for Use by Reference Only ). Items other than subpatterns that have a quantifier are omitted from the compiled pattern.
For convenience, the three most common quantifiers have single-character abbreviations:
Infinite loops can be constructed by following a subpattern that can match no characters with a quantifier that has no upper limit, for example:
Earlier versions of Perl and PCRE used to give an error at compile time for such patterns. However, as there are cases where this can be useful, such patterns are now accepted. However, if any repetition of the subpattern matches no characters, the loop is forcibly broken.
By default, the quantifiers are "greedy", that is, they match as much as possible (up to the maximum number of permitted times), without causing the remaining pattern to fail. The classic example of where this gives problems is in trying to match comments in C programs. These appear between /* and */. Within the comment, individual * and / characters can appear. An attempt to match C comments by applying the pattern.
fails, as it matches the entire string owing to the greediness of the .* item.
However, if a quantifier is followed by a question mark, it ceases to be greedy, and instead matches the minimum number of times possible, so the following pattern does the right thing with the C comments:
The meaning of the various quantifiers is not otherwise changed, only the preferred number of matches. Do not confuse this use of question mark with its use as a quantifier in its own right. As it has two uses, it can sometimes appear doubled, as in.
which matches one digit by preference, but can match two if that is the only way the remaining pattern matches.
If option ungreedy is set (an option that is not available in Perl), the quantifiers are not greedy by default, but individual ones can be made greedy by following them with a question mark. That is, it inverts the default behavior.
When a parenthesized subpattern is quantified with a minimum repeat count that is > 1 or with a limited maximum, more memory is required for the compiled pattern, in proportion to the size of the minimum or maximum.
If a pattern starts with .* or . and option dotall (equivalent to Perl option /s ) is set, thus allowing the dot to match newlines, the pattern is implicitly anchored, because whatever follows is tried against every character position in the subject string. So, there is no point in retrying the overall match at any position after the first. PCRE normally treats such a pattern as if it was preceded by \A.
In cases where it is known that the subject string contains no newlines, it is worth setting dotall to obtain this optimization, or alternatively using ^ to indicate anchoring explicitly.
However, there are some cases where the optimization cannot be used. When .* is inside capturing parentheses that are the subject of a back reference elsewhere in the pattern, a match at the start can fail where a later one succeeds. Consider, for example:
If the subject is "xyz123abc123", the match point is the fourth character. Therefore, such a pattern is not implicitly anchored.
Another case where implicit anchoring is not applied is when the leading .* is inside an atomic group. Once again, a match at the start can fail where a later one succeeds. Consider the following pattern:
It matches "ab" in the subject "aab". The use of the backtracking control verbs (*PRUNE) and (*SKIP) also disable this optimization.
When a capturing subpattern is repeated, the value captured is the substring that matched the final iteration. For example, after.
has matched "tweedledum tweedledee", the value of the captured substring is "tweedledee". However, if there are nested capturing subpatterns, the corresponding captured values can have been set in previous iterations. For example, after.
matches "aba", the value of the second captured substring is "b".
Atomic Grouping and Possessive Quantifiers.
With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") repetition, failure of what follows normally causes the repeated item to be re-evaluated to see if a different number of repeats allows the remaining pattern to match. Sometimes it is useful to prevent this, either to change the nature of the match, or to cause it to fail earlier than it otherwise might, when the author of the pattern knows that there is no point in carrying on.
Consider, for example, the pattern \d+foo when applied to the following subject line:
After matching all six digits and then failing to match "foo", the normal action of the matcher is to try again with only five digits matching item \d+, and then with four, and so on, before ultimately failing. "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides the means for specifying that once a subpattern has matched, it is not to be re-evaluated in this way.
If atomic grouping is used for the previous example, the matcher gives up immediately on failing to match "foo" the first time. The notation is a kind of special parenthesis, starting with (?> as in the following example:
This kind of parenthesis "locks up" the part of the pattern it contains once it has matched, and a failure further into the pattern is prevented from backtracking into it. Backtracking past it to previous items, however, works as normal.
An alternative description is that a subpattern of this type matches the string of characters that an identical standalone pattern would match, if anchored at the current point in the subject string.
Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as the above example can be thought of as a maximizing repeat that must swallow everything it can. So, while both \d+ and \d+? are prepared to adjust the number of digits they match to make the remaining pattern match, (?>\d+) can only match an entire sequence of digits.
Atomic groups in general can contain any complicated subpatterns, and can be nested. However, when the subpattern for an atomic group is just a single repeated item, as in the example above, a simpler notation, called a "possessive quantifier" can be used. This consists of an extra + character following a quantifier. Using this notation, the previous example can be rewritten as.
Notice that a possessive quantifier can be used with an entire group, for example:
Possessive quantifiers are always greedy; the setting of option ungreedy is ignored. They are a convenient notation for the simpler forms of an atomic group. However, there is no difference in the meaning of a possessive quantifier and the equivalent atomic group, but there can be a performance difference; possessive quantifiers are probably slightly faster.
The possessive quantifier syntax is an extension to the Perl 5.8 syntax. Jeffrey Friedl originated the idea (and the name) in the first edition of his book. Mike McCloskey liked it, so implemented it when he built the Sun Java package, and PCRE copied it from there. It ultimately found its way into Perl at release 5.10.
PCRE has an optimization that automatically "possessifies" certain simple pattern constructs. For example, the sequence A+B is treated as A++B, as there is no point in backtracking into a sequence of A:s when B must follow.
When a pattern contains an unlimited repeat inside a subpattern that can itself be repeated an unlimited number of times, the use of an atomic group is the only way to avoid some failing matches taking a long time. O padrão.
matches an unlimited number of substrings that either consist of non-digits, or digits enclosed in <>, followed by ! or ?. When it matches, it runs quickly. However, if it is applied to.
it takes a long time before reporting failure. This is because the string can be divided between the internal \D+ repeat and the external * repeat in many ways, and all must be tried. (The example uses [!?] rather than a single character at the end, as both PCRE and Perl have an optimization that allows for fast failure when a single character is used. They remember the last single character that is required for a match, and fail early if it is not present in the string.) If the pattern is changed so that it uses an atomic group, like the following, sequences of non-digits cannot be broken, and failure happens quickly:
Back References.
Outside a character class, a backslash followed by a digit > 0 (and possibly further digits) is a back reference to a capturing subpattern earlier (that is, to its left) in the pattern, provided there have been that many previous capturing left parentheses.
However, if the decimal number following the backslash is < 10, it is always taken as a back reference, and causes an error only if there are not that many capturing left parentheses in the entire pattern. That is, the parentheses that are referenced do need not be to the left of the reference for numbers < 10. A "forward back reference" of this type can make sense when a repetition is involved and the subpattern to the right has participated in an earlier iteration.
It is not possible to have a numerical "forward back reference" to a subpattern whose number is 10 or more using this syntax, as a sequence such as \50 is interpreted as a character defined in octal. For more details of the handling of digits following a backslash, see section Non-Printing Characters earlier. There is no such problem when named parentheses are used. A back reference to any subpattern is possible using named parentheses (see below).
Another way to avoid the ambiguity inherent in the use of digits following a backslash is to use the \g escape sequence. This escape must be followed by an unsigned number or a negative number, optionally enclosed in braces. The following examples are identical:
An unsigned number specifies an absolute reference without the ambiguity that is present in the older syntax. It is also useful when literal digits follow the reference. A negative number is a relative reference. Considere o seguinte exemplo:
The sequence \g is a reference to the most recently started capturing subpattern before \g, that is, it is equivalent to \2 in this example. Similarly, \g would be equivalent to \1. The use of relative references can be helpful in long patterns, and also in patterns that are created by joining fragments containing references within themselves.
A back reference matches whatever matched the capturing subpattern in the current subject string, rather than anything matching the subpattern itself (section Subpattern as Subroutines describes a way of doing that). So, the following pattern matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility":
If caseful matching is in force at the time of the back reference, the case of letters is relevant. For example, the following matches "rah rah" and "RAH RAH", but not "RAH rah", although the original capturing subpattern is matched caselessly:
There are many different ways of writing back references to named subpatterns. The syntax \k and the Perl syntax \k<name> or \k'name' are supported, as is the Python syntax (?P=name) . The unified back reference syntax in Perl 5.10, in which \g can be used for both numeric and named references, is also supported. The previous example can be rewritten in the following ways:
A subpattern that is referenced by name can appear in the pattern before or after the reference.
There can be more than one back reference to the same subpattern. If a subpattern has not been used in a particular match, any back references to it always fails. For example, the following pattern always fails if it starts to match "a" rather than "bc":
As there can be many capturing parentheses in a pattern, all digits following the backslash are taken as part of a potential back reference number. If the pattern continues with a digit character, some delimiter must be used to terminate the back reference. If option extended is set, this can be whitespace. Otherwise an empty comment (see section Comments ) can be used.
Recursive Back References.
A back reference that occurs inside the parentheses to which it refers fails when the subpattern is first used, so, for example, (a\1) never matches. However, such references can be useful inside repeated subpatterns. For example, the following pattern matches any number of "a"s and also "aba", "ababbaa", and so on:
At each iteration of the subpattern, the back reference matches the character string corresponding to the previous iteration. In order for this to work, the pattern must be such that the first iteration does not need to match the back reference. This can be done using alternation, as in the example above, or by a quantifier with a minimum of zero.
Back references of this type cause the group that they reference to be treated as an atomic group. Once the whole group has been matched, a subsequent matching failure cannot cause backtracking into the middle of the group.
Assertions.
An assertion is a test on the characters following or preceding the current matching point that does not consume any characters. The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^, and $ are described in the previous sections.
More complicated assertions are coded as subpatterns. There are two kinds: those that look ahead of the current position in the subject string, and those that look behind it. An assertion subpattern is matched in the normal way, except that it does not cause the current matching position to be changed.
Assertion subpatterns are not capturing subpatterns. If such an assertion contains capturing subpatterns within it, these are counted for the purposes of numbering the capturing subpatterns in the whole pattern. However, substring capturing is done only for positive assertions. (Perl sometimes, but not always, performs capturing in negative assertions.)
If a positive assertion containing one or more capturing subpatterns succeeds, but failure to match later in the pattern causes backtracking over this assertion, the captures within the assertion are reset only if no higher numbered captures are already set. This is, unfortunately, a fundamental limitation of the current implementation, and as PCRE1 is now in maintenance-only status, it is unlikely ever to change.
For compatibility with Perl, assertion subpatterns can be repeated. However, it makes no sense to assert the same thing many times, the side effect of capturing parentheses can occasionally be useful. In practice, there are only three cases:
If the quantifier is , the assertion is never obeyed during matching. However, it can contain internal capturing parenthesized groups that are called from elsewhere through the subroutine mechanism.
If quantifier is , where n > 0, it is treated as if it was . At runtime, the remaining pattern match is tried with and without the assertion, the order depends on the greediness of the quantifier.
If the minimum repetition is > 0, the quantifier is ignored. The assertion is obeyed only once when encountered during matching.
Lookahead assertions start with (?= for positive assertions and (?! for negative assertions. For example, the following matches a word followed by a semicolon, but does not include the semicolon in the match:
The following matches any occurrence of "foo" that is not followed by "bar":
Notice that the apparently similar pattern.
does not find an occurrence of "bar" that is preceded by something other than "foo". It finds any occurrence of "bar" whatsoever, as the assertion (?!foo) is always true when the next three characters are "bar". A lookbehind assertion is needed to achieve the other effect.
If you want to force a matching failure at some point in a pattern, the most convenient way to do it is with (?!), as an empty string always matches. So, an assertion that requires there is not to be an empty string must always fail. The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
Lookbehind assertions start with (?<= for positive assertions and (?<! for negative assertions. For example, the following finds an occurrence of "bar" that is not preceded by "foo":
The contents of a lookbehind assertion are restricted such that all the strings it matches must have a fixed length. However, if there are many top-level alternatives, they do not all have to have the same fixed length. Thus, the following is permitted:
The following causes an error at compile time:
Branches that match different length strings are permitted only at the top-level of a lookbehind assertion. This is an extension compared with Perl, which requires all branches to match the same length of string. An assertion such as the following is not permitted, as its single top-level branch can match two different lengths:
However, it is acceptable to PCRE if rewritten to use two top-level branches:
Sometimes the escape sequence \K (see above) can be used instead of a lookbehind assertion to get round the fixed-length restriction.
The implementation of lookbehind assertions is, for each alternative, to move the current position back temporarily by the fixed length and then try to match. If there are insufficient characters before the current position, the assertion fails.
In a UTF mode, PCRE does not allow the \C escape (which matches a single data unit even in a UTF mode) to appear in lookbehind assertions, as it makes it impossible to calculate the length of the lookbehind. The \X and \R escapes, which can match different numbers of data units, are not permitted either.
"Subroutine" calls (see below), such as (?2) or (?&X), are permitted in lookbehinds, as long as the subpattern matches a fixed-length string. Recursion, however, is not supported.
Possessive quantifiers can be used with lookbehind assertions to specify efficient matching of fixed-length strings at the end of subject strings. Consider the following simple pattern when applied to a long string that does not match:
As matching proceeds from left to right, PCRE looks for each "a" in the subject and then sees if what follows matches the remaining pattern. If the pattern is specified as.
the initial .* matches the entire string at first. However, when this fails (as there is no following "a"), it backtracks to match all but the last character, then all but the last two characters, and so on. Once again the search for "a" covers the entire string, from right to left, so we are no better off. However, if the pattern is written as.
there can be no backtracking for the .*+ item; it can match only the entire string. The subsequent lookbehind assertion does a single test on the last four characters. If it fails, the match fails immediately. For long strings, this approach makes a significant difference to the processing time.
Using Multiple Assertions.
Many assertions (of any sort) can occur in succession. For example, the following matches "foo" preceded by three digits that are not "999":
Notice that each of the assertions is applied independently at the same point in the subject string. First there is a check that the previous three characters are all digits, and then there is a check that the same three characters are not "999". This pattern does not match "foo" preceded by six characters, the first of which are digits and the last three of which are not "999". For example, it does not match "123abcfoo". A pattern to do that is the following:
This time the first assertion looks at the preceding six characters, checks that the first three are digits, and then the second assertion checks that the preceding three characters are not "999".
Assertions can be nested in any combination. For example, the following matches an occurrence of "baz" that is preceded by "bar", which in turn is not preceded by "foo":
The following pattern matches "foo" preceded by three digits and any three characters that are not "999":
Conditional Subpatterns.
It is possible to cause the matching process to obey a subpattern conditionally or to choose between two alternative subpatterns, depending on the result of an assertion, or whether a specific capturing subpattern has already been matched. The following are the two possible forms of conditional subpattern:
If the condition is satisfied, the yes-pattern is used, otherwise the no-pattern (if present). If more than two alternatives exist in the subpattern, a compile-time error occurs. Each of the two alternatives can itself contain nested subpatterns of any form, including conditional subpatterns; the restriction to two alternatives applies only at the level of the condition. The following pattern fragment is an example where the alternatives are complex:
There are four kinds of condition: references to subpatterns, references to recursion, a pseudo-condition called DEFINE, and assertions.
Checking for a Used Subpattern By Number.
If the text between the parentheses consists of a sequence of digits, the condition is true if a capturing subpattern of that number has previously matched. If more than one capturing subpattern with the same number exists (see section Duplicate Subpattern Numbers earlier), the condition is true if any of them have matched. An alternative notation is to precede the digits with a plus or minus sign. In this case, the subpattern number is relative rather than absolute. The most recently opened parentheses can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside loops, it can also make sense to refer to subsequent groups. The next parentheses to be opened can be referenced as (?(+1), and so on. (The value zero in any of these forms is not used; it provokes a compile-time error.)
Consider the following pattern, which contains non-significant whitespace to make it more readable (assume option extended ) and to divide it into three parts for ease of discussion:
The first part matches an optional opening parenthesis, and if that character is present, sets it as the first captured substring. The second part matches one or more characters that are not parentheses. The third part is a conditional subpattern that tests whether the first set of parentheses matched or not. If they did, that is, if subject started with an opening parenthesis, the condition is true, and so the yes-pattern is executed and a closing parenthesis is required. Otherwise, as no-pattern is not present, the subpattern matches nothing. That is, this pattern matches a sequence of non-parentheses, optionally enclosed in parentheses.
If this pattern is embedded in a larger one, a relative reference can be used:
This makes the fragment independent of the parentheses in the larger pattern.
Checking for a Used Subpattern By Name.
Perl uses the syntax (?(<name>). ) or (?('name'). ) to test for a used subpattern by name. For compatibility with earlier versions of PCRE, which had this facility before Perl, the syntax (?(name). ) is also recognized.
Rewriting the previous example to use a named subpattern gives:
If the name used in a condition of this kind is a duplicate, the test is applied to all subpatterns of the same name, and is true if any one of them has matched.
Checking for Pattern Recursion.
If the condition is the string (R), and there is no subpattern with the name R, the condition is true if a recursive call to the whole pattern or any subpattern has been made. If digits or a name preceded by ampersand follow the letter R, for example:
the condition is true if the most recent recursion is into a subpattern whose number or name is given. This condition does not check the entire recursion stack. If the name used in a condition of this kind is a duplicate, the test is applied to all subpatterns of the same name, and is true if any one of them is the most recent recursion.
At "top-level", all these recursion test conditions are false. The syntax for recursive patterns is described below.
Defining Subpatterns for Use By Reference Only.
If the condition is the string (DEFINE), and there is no subpattern with the name DEFINE, the condition is always false. In this case, there can be only one alternative in the subpattern. It is always skipped if control reaches this point in the pattern. The idea of DEFINE is that it can be used to define "subroutines" that can be referenced from elsewhere. (The use of subroutines is described below.) For example, a pattern to match an IPv4 address, such as "192.168.23.245", can be written like this (ignore whitespace and line breaks):
The first part of the pattern is a DEFINE group inside which is a another group named "byte" is defined. This matches an individual component of an IPv4 address (a number < 256). When matching takes place, this part of the pattern is skipped, as DEFINE acts like a false condition. The remaining pattern uses references to the named group to match the four dot-separated components of an IPv4 address, insisting on a word boundary at each end.
If the condition is not in any of the above formats, it must be an assertion. This can be a positive or negative lookahead or lookbehind assertion. Consider the following pattern, containing non-significant whitespace, and with the two alternatives on the second line:
The condition is a positive lookahead assertion that matches an optional sequence of non-letters followed by a letter. That is, it tests for the presence of at least one letter in the subject. If a letter is found, the subject is matched against the first alternative, otherwise it is matched against the second. This pattern matches strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
There are two ways to include comments in patterns that are processed by PCRE. In both cases, the start of the comment must not be in a character class, or in the middle of any other sequence of related characters such as (?: or a subpattern name or number. The characters that make up a comment play no part in the pattern matching.
The sequence (?# marks the start of a comment that continues up to the next closing parenthesis. Nested parentheses are not permitted. If option PCRE_EXTENDED is set, an unescaped # character also introduces a comment, which in this case continues to immediately after the next newline character or character sequence in the pattern. Which characters are interpreted as newlines is controlled by the options passed to a compiling function or by a special sequence at the start of the pattern, as described in section Newline Conventions earlier.
Notice that the end of this type of comment is a literal newline sequence in the pattern; escape sequences that happen to represent a newline do not count. For example, consider the following pattern when extended is set, and the default newline convention is in force:
On encountering character #, pcre_compile() skips along, looking for a newline in the pattern. The sequence \n is still literal at this stage, so it does not terminate the comment. Only a character with code value 0x0a (the default newline) does so.
Recursive Patterns.
Consider the problem of matching a string in parentheses, allowing for unlimited nested parentheses. Without the use of recursion, the best that can be done is to use a pattern that matches up to some fixed depth of nesting. It is not possible to handle an arbitrary nesting depth.
For some time, Perl has provided a facility that allows regular expressions to recurse (among other things). It does this by interpolating Perl code in the expression at runtime, and the code can refer to the expression itself. A Perl pattern using code interpolation to solve the parentheses problem can be created like this:
Item (?p ) interpolates Perl code at runtime, and in this case refers recursively to the pattern in which it appears.
Obviously, PCRE cannot support the interpolation of Perl code. Instead, it supports special syntax for recursion of the entire pattern, and for individual subpattern recursion. After its introduction in PCRE and Python, this kind of recursion was later introduced into Perl at release 5.10.
A special item that consists of (? followed by a number > 0 and a closing parenthesis is a recursive subroutine call of the subpattern of the given number, if it occurs inside that subpattern. (If not, it is a non-recursive subroutine call, which is described in the next section.) The special item (?R) or (?0) is a recursive call of the entire regular expression.
This PCRE pattern solves the nested parentheses problem (assume that option extended is set so that whitespace is ignored):
First it matches an opening parenthesis. Then it matches any number of substrings, which can either be a sequence of non-parentheses or a recursive match of the pattern itself (that is, a correctly parenthesized substring). Finally there is a closing parenthesis. Notice the use of a possessive quantifier to avoid backtracking into sequences of non-parentheses.
If this was part of a larger pattern, you would not want to recurse the entire pattern, so instead you can use:
The pattern is here within parentheses so that the recursion refers to them instead of the whole pattern.
In a larger pattern, keeping track of parenthesis numbers can be tricky. This is made easier by the use of relative references. Instead of (?1) in the pattern above, you can write (?-2) to refer to the second most recently opened parentheses preceding the recursion. That is, a negative number counts capturing parentheses leftwards from the point at which it is encountered.
It is also possible to refer to later opened parentheses, by writing references such as (?+2). However, these cannot be recursive, as the reference is not inside the parentheses that are referenced. They are always non-recursive subroutine calls, as described in the next section.
An alternative approach is to use named parentheses instead. The Perl syntax for this is (?&name). The earlier PCRE syntax (?P>name) is also supported. We can rewrite the above example as follows:
If there is more than one subpattern with the same name, the earliest one is used.
This particular example pattern that we have studied contains nested unlimited repeats, and so the use of a possessive quantifier for matching strings of non-parentheses is important when applying the pattern to strings that do not match. For example, when this pattern is applied to.
it gives "no match" quickly. However, if a possessive quantifier is not used, the match runs for a long time, as there are so many different ways the + and * repeats can carve up the subject, and all must be tested before failure can be reported.
At the end of a match, the values of capturing parentheses are those from the outermost level. If the pattern above is matched against.
the value for the inner capturing parentheses (numbered 2) is "ef", which is the last value taken on at the top-level. If a capturing subpattern is not matched at the top level, its final captured value is unset, even if it was (temporarily) set at a deeper level during the matching process.
Do not confuse item (?R) with condition (R), which tests for recursion. Consider the following pattern, which matches text in angle brackets, allowing for arbitrary nesting. Only digits are allowed in nested brackets (that is, when recursing), while any characters are permitted at the outer level.
Here (?(R) is the start of a conditional subpattern, with two different alternatives for the recursive and non-recursive cases. Item (?R) is the actual recursive call.
Differences in Recursion Processing between PCRE and Perl.
Recursion processing in PCRE differs from Perl in two important ways. In PCRE (like Python, but unlike Perl), a recursive subpattern call is always treated as an atomic group. That is, once it has matched some of the subject string, it is never re-entered, even if it contains untried alternatives and there is a subsequent matching failure. This can be illustrated by the following pattern, which means to match a palindromic string containing an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
The idea is that it either matches a single character, or two identical characters surrounding a subpalindrome. In Perl, this pattern works; in PCRE it does not work if the pattern is longer than three characters. Consider the subject string "abcba".
At the top level, the first character is matched, but as it is not at the end of the string, the first alternative fails, the second alternative is taken, and the recursion kicks in. The recursive call to subpattern 1 successfully matches the next character ("b"). (Notice that the beginning and end of line tests are not part of the recursion.)
Back at the top level, the next character ("c") is compared with what subpattern 2 matched, which was "a". This fails. As the recursion is treated as an atomic group, there are now no backtracking points, and so the entire match fails. (Perl can now re-enter the recursion and try the second alternative.) However, if the pattern is written with the alternatives in the other order, things are different:
This time, the recursing alternative is tried first, and continues to recurse until it runs out of characters, at which point the recursion fails. But this time we have another alternative to try at the higher level. That is the significant difference: in the previous case the remaining alternative is at a deeper recursion level, which PCRE cannot use.
To change the pattern so that it matches all palindromic strings, not only those with an odd number of characters, it is tempting to change the pattern to this:
Again, this works in Perl, but not in PCRE, and for the same reason. When a deeper recursion has matched a single character, it cannot be entered again to match an empty string. The solution is to separate the two cases, and write out the odd and even cases as alternatives at the higher level:
If you want to match typical palindromic phrases, the pattern must ignore all non-word characters, which can be done as follows:
If run with option caseless , this pattern matches phrases such as "A man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Notice the use of the possessive quantifier *+ to avoid backtracking into sequences of non-word characters. Without this, PCRE takes much longer (10 times or more) to match typical phrases, and Perl takes so long that you think it has gone into a loop.
The palindrome-matching patterns above work only if the subject string does not start with a palindrome that is shorter than the entire string. For example, although "abcba" is correctly matched, if the subject is "ababa", PCRE finds palindrome "aba" at the start, and then fails at top level, as the end of the string does not follow. Once again, it cannot jump back into the recursion to try other alternatives, so the entire match fails.
The second way in which PCRE and Perl differ in their recursion processing is in the handling of captured values. In Perl, when a subpattern is called recursively or as a subpattern (see the next section), it has no access to any values that were captured outside the recursion. In PCRE these values can be referenced. Consider the following pattern:
In PCRE, it matches "bab". The first capturing parentheses match "b", then in the second group, when the back reference \1 fails to match "b", the second alternative matches "a", and then recurses. In the recursion, \1 does now match "b" and so the whole match succeeds. In Perl, the pattern fails to match because inside the recursive call \1 cannot access the externally set value.
Subpatterns as Subroutines.
If the syntax for a recursive subpattern call (either by number or by name) is used outside the parentheses to which it refers, it operates like a subroutine in a programming language. The called subpattern can be defined before or after the reference. A numbered reference can be absolute or relative, as in the following examples:
An earlier example pointed out that the following pattern matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility":
If instead the following pattern is used, it matches "sense and responsibility" and the other two strings:
Another example is provided in the discussion of DEFINE earlier.
All subroutine calls, recursive or not, are always treated as atomic groups. That is, once a subroutine has matched some of the subject string, it is never re-entered, even if it contains untried alternatives and there is a subsequent matching failure. Any capturing parentheses that are set during the subroutine call revert to their previous values afterwards.
Processing options such as case-independence are fixed when a subpattern is defined, so if it is used as a subroutine, such options cannot be changed for different calls. For example, the following pattern matches "abcabc" but not "abcABC", as the change of processing option does not affect the called subpattern:
Oniguruma Subroutine Syntax.
For compatibility with Oniguruma, the non-Perl syntax \g followed by a name or a number enclosed either in angle brackets or single quotes, is alternative syntax for referencing a subpattern as a subroutine, possibly recursively. Here follows two of the examples used above, rewritten using this syntax:
PCRE supports an extension to Oniguruma: if a number is preceded by a plus or minus sign, it is taken as a relative reference, for example:
Notice that \g (Perl syntax) and \g<. & gt; (Oniguruma syntax) are not synonymous. The former is a back reference; the latter is a subroutine call.
Backtracking Control.
Perl 5.10 introduced some "Special Backtracking Control Verbs", which are still described in the Perl documentation as "experimental and subject to change or removal in a future version of Perl". It goes on to say: "Their usage in production code should be noted to avoid problems during upgrades." The same remarks apply to the PCRE features described in this section.
The new verbs make use of what was previously invalid syntax: an opening parenthesis followed by an asterisk. They are generally of the form (*VERB) or (*VERB:NAME). Some can take either form, possibly behaving differently depending on whether a name is present. A name is any sequence of characters that does not include a closing parenthesis. The maximum name length is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit libraries. If the name is empty, that is, if the closing parenthesis immediately follows the colon, the effect is as if the colon was not there. Any number of these verbs can occur in a pattern.
The behavior of these verbs in repeated groups, assertions, and in subpatterns called as subroutines (whether or not recursively) is described below.
Optimizations That Affect Backtracking Verbs.
PCRE contains some optimizations that are used to speed up matching by running some checks at the start of each match attempt. For example, it can know the minimum length of matching subject, or that a particular character must be present. When one of these optimizations bypasses the running of a match, any included backtracking verbs are not processed. processed. You can suppress the start-of-match optimizations by setting option no_start_optimize when calling compile/2 or run/3 , or by starting the pattern with (*NO_START_OPT).
Experiments with Perl suggest that it too has similar optimizations, sometimes leading to anomalous results.
Verbs That Act Immediately.
The following verbs act as soon as they are encountered. They must not be followed by a name.
This verb causes the match to end successfully, skipping the remainder of the pattern. However, when it is inside a subpattern that is called as a subroutine, only that subpattern is ended successfully. Matching then continues at the outer level. If (*ACCEPT) is triggered in a positive assertion, the assertion succeeds; in a negative assertion, the assertion fails.
If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For example, the following matches "AB", "AAD", or "ACD". When it matches "AB", "B" is captured by the outer parentheses.
The following verb causes a matching failure, forcing backtracking to occur. It is equivalent to (?!) but easier to read.
The Perl documentation states that it is probably useful only when combined with (?<>) or (??<>). Those are Perl features that are not present in PCRE.
A match with the string "aaaa" always fails, but the callout is taken before each backtrack occurs (in this example, 10 times).
Recording Which Path Was Taken.
The main purpose of this verb is to track how a match was arrived at, although it also has a secondary use in with advancing the match starting point (see (*SKIP) below).
In Erlang, there is no interface to retrieve a mark with run/2,3 , so only the secondary purpose is relevant to the Erlang programmer.
The rest of this section is therefore deliberately not adapted for reading by the Erlang programmer, but the examples can help in understanding NAMES as they can be used by (*SKIP).
A name is always required with this verb. There can be as many instances of (*MARK) as you like in a pattern, and their names do not have to be unique.
When a match succeeds, the name of the last encountered (*MARK:NAME), (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the caller as described in section "Extra data for pcre_exec() " in the pcreapi documentation. In the following example of pcretest output, the /K modifier requests the retrieval and outputting of (*MARK) data:
The (*MARK) name is tagged with "MK:" in this output, and in this example it indicates which of the two alternatives matched. This is a more efficient way of obtaining this information than putting each alternative in its own capturing parentheses.
If a verb with a name is encountered in a positive assertion that is true, the name is recorded and passed back if it is the last encountered. This does not occur for negative assertions or failing positive assertions.
After a partial match or a failed match, the last encountered name in the entire match process is returned, for example:
Notice that in this unanchored example, the mark is retained from the match attempt that started at letter "X" in the subject. Subsequent match attempts starting at "P" and then with an empty string do not get as far as the (*MARK) item, nevertheless do not reset it.
Verbs That Act after Backtracking.
The following verbs do nothing when they are encountered. Matching continues with what follows, but if there is no subsequent match, causing a backtrack to the verb, a failure is forced. That is, backtracking cannot pass to the left of the verb. However, when one of these verbs appears inside an atomic group or an assertion that is true, its effect is confined to that group, as once the group has been matched, there is never any backtracking into it. In this situation, backtracking can "jump back" to the left of the entire atomic group or assertion. (Remember also, as stated above, that this localization also applies in subroutine calls.)
These verbs differ in exactly what kind of failure occurs when backtracking reaches them. The behavior described below is what occurs when the verb is not in a subroutine or an assertion. Subsequent sections cover these special cases.
The following verb, which must not be followed by a name, causes the whole match to fail outright if there is a later matching failure that causes backtracking to reach it. Even if the pattern is unanchored, no further attempts to find a match by advancing the starting point take place.
If (*COMMIT) is the only backtracking verb that is encountered, once it has been passed, run/2,3 is committed to find a match at the current starting point, or not at all, for example:
This matches "xxaab" but not "aacaab". It can be thought of as a kind of dynamic anchor, or "I've started, so I must finish". The name of the most recently passed (*MARK) in the path is passed back when (*COMMIT) forces a match failure.
If more than one backtracking verb exists in a pattern, a different one that follows (*COMMIT) can be triggered first, so merely passing (*COMMIT) during a match does not always guarantee that a match must be at this starting point.
Notice that (*COMMIT) at the start of a pattern is not the same as an anchor, unless the PCRE start-of-match optimizations are turned off, as shown in the following example:
For this pattern, PCRE knows that any match must start with "a", so the optimization skips along the subject to "a" before applying the pattern to the first set of data. The match attempt then succeeds. In the second call the no_start_optimize disables the optimization that skips along to the first character. The pattern is now applied starting at "x", and so the (*COMMIT) causes the match to fail without trying any other starting points.
The following verb causes the match to fail at the current starting position in the subject if there is a later matching failure that causes backtracking to reach it:
If the pattern is unanchored, the normal "bumpalong" advance to the next starting character then occurs. Backtracking can occur as usual to the left of (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but if there is no match to the right, backtracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alternative to an atomic group or possessive quantifier, but there are some uses of (*PRUNE) that cannot be expressed in any other way. In an anchored pattern, (*PRUNE) has the same effect as (*COMMIT).
The behavior of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is remembered for passing back to the caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
The fact that (*PRUNE:NAME) remembers the name is useless to the Erlang programmer, as names cannot be retrieved.
The following verb, when specified without a name, is like (*PRUNE), except that if the pattern is unanchored, the "bumpalong" advance is not to the next character, but to the position in the subject where (*SKIP) was encountered.
(*SKIP) signifies that whatever text was matched leading up to it cannot be part of a successful match. Considerar:
If the subject is "aaaac. ", after the first match attempt fails (starting at the first character in the string), the starting point skips on to start the next attempt at "c". Notice that a possessive quantifier does not have the same effect as this example; although it would suppress backtracking during the first match attempt, the second attempt would start at the second character instead of skipping on to "c".
When (*SKIP) has an associated name, its behavior is modified:
When this is triggered, the previous path through the pattern is searched for the most recent (*MARK) that has the same name. If one is found, the "bumpalong" advance is to the subject position that corresponds to that (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a matching name is found, (*SKIP) is ignored.
Notice that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME).
The following verb causes a skip to the next innermost alternative when backtracking reaches it. That is, it cancels any further backtracking within the current alternative.
The verb name comes from the observation that it can be used for a pattern-based if-then-else block:
If the COND1 pattern matches, FOO is tried (and possibly further items after the end of the group if FOO succeeds). On failure, the matcher skips to the second alternative and tries COND2, without backtracking into COND1. If that succeeds and BAR fails, COND3 is tried. If BAZ then fails, there are no more alternatives, so there is a backtrack to whatever came before the entire group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
The behavior of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is remembered for passing back to the caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
The fact that (*THEN:NAME) remembers the name is useless to the Erlang programmer, as names cannot be retrieved.
A subpattern that does not contain a | character is just a part of the enclosing alternative; it is not a nested alternation with only one alternative. The effect of (*THEN) extends beyond such a subpattern to the enclosing alternative. Consider the following pattern, where A, B, and so on, are complex pattern fragments that do not contain any | characters at this level:
If A and B are matched, but there is a failure in C, matching does not backtrack into A; instead it moves to the next alternative, that is, D. However, if the subpattern containing (*THEN) is given an alternative, it behaves differently:
The effect of (*THEN) is now confined to the inner subpattern. After a failure in C, matching moves to (*FAIL), which causes the whole subpattern to fail, as there are no more alternatives to try. In this case, matching does now backtrack into A.
Notice that a conditional subpattern is not considered as having two alternatives, as only one is ever used. That is, the | character in a conditional subpattern has a different meaning. Ignoring whitespace, consider:
If the subject is "ba", this pattern does not match. As .*? is ungreedy, it initially matches zero characters. The condition (?=a) then fails, the character "b" is matched, but "c" is not. At this point, matching does not backtrack to .*? as can perhaps be expected from the presence of the | personagem. The conditional subpattern is part of the single alternative that comprises the whole pattern, and so the match fails. (If there was a backtrack into .*?, allowing it to match "b", the match would succeed.)
The verbs described above provide four different "strengths" of control when subsequent matching fails:
(*THEN) is the weakest, carrying on the match at the next alternative.
(*PRUNE) comes next, fails the match at the current starting position, but allows an advance to the next character (for an unanchored pattern).
(*SKIP) is similar, except that the advance can be more than one character.
(*COMMIT) is the strongest, causing the entire match to fail.
More than One Backtracking Verb.
If more than one backtracking verb is present in a pattern, the one that is backtracked onto first acts. For example, consider the following pattern, where A, B, and so on, are complex pattern fragments:
If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes the next alternative (ABD) to be tried. This behavior is consistent, but is not always the same as in Perl. It means that if two or more backtracking verbs appear in succession, the last of them has no effect. Considere o seguinte exemplo:
If there is a matching failure to the right, backtracking onto (*PRUNE) causes it to be triggered, and its action is taken. There can never be a backtrack onto (*COMMIT).
Backtracking Verbs in Repeated Groups.
PCRE differs from Perl in its handling of backtracking verbs in repeated groups. For example, consider:
If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in the second repeat of the group acts.
Backtracking Verbs in Assertions.
(*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
(*ACCEPT) in a positive assertion causes the assertion to succeed without any further processing. In a negative assertion, (*ACCEPT) causes the assertion to fail without any further processing.
The other backtracking verbs are not treated specially if they appear in a positive assertion. In particular, (*THEN) skips to the next alternative in the innermost enclosing group that has alternations, regardless if this is within the assertion.
Negative assertions are, however, different, to ensure that changing a positive assertion into a negative assertion changes its result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true, without considering any further alternative branches in the assertion. Backtracking into (*THEN) causes it to skip to the next enclosing alternative within the assertion (the normal behavior), but if the assertion does not have such an alternative, (*THEN) behaves like (*PRUNE).
Backtracking Verbs in Subroutines.
These behaviors occur regardless if the subpattern is called recursively. The treatment of subroutines in Perl is different in some cases.
(*FAIL) in a subpattern called as a subroutine has its normal effect: it forces an immediate backtrack.
(*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to succeed without any further processing. Matching then continues after the subroutine call.
(*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause the subroutine match to fail.
(*THEN) skips to the next alternative in the innermost enclosing group within the subpattern that has alternatives. If there is no such group within the subpattern, (*THEN) causes the subroutine match to fail.
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This paper describes the telecommunications unit Erlang, and its application in teletraffic theory. Online Erlang Traffic Calculators are also hosted at this Web site and can be used free of charge now.
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Erlang - a unit of traffic.
For example, if a group of user made 30 calls in one hour, and each call had an average call duration of 5 minutes, then the number of Erlangs this represents is worked out as follows:
Erlang traffic measurements are made in order to help telecommunications network designers understand traffic patterns within their voice networks. This is essential if they are to successfully design their network topology and establish the necessary trunk group sizes.
Erlang traffic measurements or estimates can be used to work out how many lines are required between a telephone system and a central office (PSTN exchange lines), or between multiple network locations.
Erlang traffic models.
The main Erlang traffic model are listed below, with links to the free online calculators on this Web site: Erlang B.
Leitura adicional.
A. K. Erlang - a tribute.
In 1909, he published his first work: The Theory of Probabilities and Telephone Conversations . He gained worldwide recognition for his work, and his formula was accepted for use by the General Post Office in the UK.
Erlang never married. He worked for the Copenhagen Telephone Company for twenty years, until his death in 1929. During the 1940s, the Erlang became the accepted unit of telecommunication traffic measurement, and his formula is still used today in the design of modern telecommunications networks.
Conclusão.
Why not try our Telecom Design Forum? It is an interactive newsgroup which you can use to exchange ideas about telecommunications design, and our products.

Erlang forex

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Haskell vs Erlang for web services.
I am looking to start an experimental project using a functional language and am trying to decide beween Erlang and Haskell, and both have some points that I really like.
I like Haskell's strong type system and purity. I have a feeling it will make it easier to write really reliable code. And I think that the power of Haskell will make some of what I want to do much easier.
On the minus side I get the feeling that some of the Frameworks for doing web stuff on Haskell such as Yesod are not as advanced as their Erlang counter parts.
I rather like the Erlang approach to threads and to fault tolerance. I have a feeling that the scalability of Erlang could be a major plus.
Which leads to to my question, what has people's experience been in implementing web application backends in both Haskell and Erlang. Are there packages for Haskell to provide some of the lightweight threads and actors that one has in Erlang?
The only question I have is what is your web service doing? If the web service is truly a functional problem, then Haskell will be a better fit.
Erlang isn't necessarily a functional language. It's a procedural language with a very strong execution model for massively parallel systems. It was designed for the telecom industry, and it would definitely make an excellent fit for responding to web service requests.
See this page* for an overview of the differences between procedural and functional programming. (Apologies in advance for the ugly black on cyan page).
If your web service is doing a fair amount of pattern matching and applying rules, then Haskel is your choice. If you just want a scalable infrastructure that isn't too different from the languages you probably already know, choose Erlang.
(* link via Wayback machine. Original file has been removed)
Between the two you mention, definitely Haskell is academic, while Erlang is used in real-life high-scalability projects. So of the two for web services I'd choose Erlang.
But I'd say you have third choice: Scala , a language that is heavily influenced by both Haskell and Erlang. It's used to build top notch web services like Twitter or Foursquare. There is even Lift , a web framework inspired by Rails and Django, although with a bit different, more functional approach. Foursquare is using Lift.
Usually i say: "learn things as far as possible from your zone of comfort, it will make you a better programmer even if you never use it in practice".
In this case, that could probably mean Haskell; but Erlang is not only becoming almost socially acceptable; but the main points (light processes, message passing, huge scalability) are coming on many other 'practical' platforms, so the lessons learned have big and immediate applicability on more 'real' work.
my advice: if it's for fun, do Haskell. if it's for training, go Erlang.