PHP: Hypertext Preprocessor

分隔符

当使用 PCRE 函数的时候，模式需要由分隔符闭合包裹。分隔符可以是任意非字母数字、非反斜线、非空白字符。静默忽略合法分隔符之前的空白字符。

经常使用的分隔符是正斜线(/)、hash符号(#) 以及取反符号(~)。下面的例子都是使用合法分隔符的模式。

/foo bar/
#^[^0-9]$#
+php+
%[a-zA-Z0-9_-]%

It is also possible to use bracket style delimiters where the opening and closing brackets are the starting and ending delimiter, respectively. (), {}, [] and <> are all valid bracket style delimiter pairs.

(this [is] a (pattern))
{this [is] a (pattern)}
[this [is] a (pattern)]
<this [is] a (pattern)>

Bracket style delimiters do not need to be escaped when they are used as meta characters within the pattern, but as with other delimiters they must be escaped when they are used as literal characters.

如果需要在正则模式内匹配分隔符，必须使用反斜线转义。如果分隔符经常在正则模式内出现，最好使用其他分隔符以便提高可读性。

/http:\/\//
#http://#

需要将一个字符串放入模式中使用时，可以用 preg_quote() 函数对其进行转义，它的第二个参数(可选)可以用于指定需要被转义的分隔符。

可以在结束分隔符后面增加模式修饰符。下面的例子是一个大小写不敏感的匹配：

#[a-z]#i

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Pedro Gimeno ¶

9 years ago

Note that bracket style opening and closing delimiters aren't a 100% problem-free solution, as they need to be escaped when they aren't in matching pairs within the expression. That mismatch can happen when they appear inside character classes [...], as most meta-characters lose their special meaning. Consider these examples:

<?php
  preg_match('{[{]}', ''); // Warning: preg_match(): No ending matching delimiter '}'
  preg_match('{[}]}', ''); // Warning: preg_match(): Unknown modifier ']'
  preg_match('{[}{]}', ''); // Warning: preg_match(): Unknown modifier ']'
?>

Escaping them solves it:

<?php
  preg_match('{[\{]}', ''); // OK
  preg_match('{[}]}', ''); // OK
  preg_match('{[\}\{]}', ''); // OK
?>

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Revo ¶

6 years ago

Note that angle brackets `<>` shouldn't be used as delimiters whenever you will have to invoke advanced clusters like atomic groups or lookbehinds because their including angle bracket doesn't come in pair and escaping doesn't help either.

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Munin ¶

9 years ago

preg_match('{[}]}', ''); // Warning: preg_match(): Unknown modifier ']'

preg_match('{[\}]}', ''); // OK