PHP: Hypertext Preprocessor

Delimiters

When using the PCRE functions, it is required that the pattern is enclosed by delimiters. A delimiter can be any non-alphanumeric, non-backslash, non-whitespace character. Leading whitespace before a valid delimiter is silently ignored.

Often used delimiters are forward slashes (/), hash signs (#) and tildes (~). The following are all examples of valid delimited patterns.

/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.

If the delimiter needs to be matched inside the pattern it must be escaped using a backslash. If the delimiter appears often inside the pattern, it is a good idea to choose another delimiter in order to increase readability.

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

The preg_quote() function may be used to escape a string for injection into a pattern and its optional second parameter may be used to specify the delimiter to be escaped.

You may add pattern modifiers after the ending delimiter. The following is an example of case-insensitive matching:

#[a-z]#i

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User Contributed Notes 3 notes

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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 ¶

5 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

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