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OverviewRegular expressions (often shortened to just regex) are patterns that describe a set of strings. Regular expressions are constructed analogously to arithmetic expressions, by using various operators to combine smaller expressions. They are a very powerful tool for matching or replacing text. PHP has a function-oriented interface to regular expressions, as opposed to Perl, where regular expressions are implemented at the language level. PHP supports two kinds of regular expressions out of the box: POSIX Extended and Perl-compatible. The functions for both are similar, but Perl-compatible expressions support many more options and are considered to be faster and more powerful in general.
POSIX Extended Regular Expression FunctionsThese functions all take a regular expression string as their first argument. PHP uses the POSIX extended regular expressions as defined by POSIX 1003.2. For a full description of POSIX regular expressions, see the regex man pages included in the regex directory in the PHP distribution. These man pages are usually in section 7, so to view them run one of the following commands (depending on your system):
man 7 regex
man -s 7 regex
A basic lesson on POSIX regular expressions is available at http://www.delorie.com/gnu/docs/rx/rx_3.html, and a tutorial-style introduction can be found at http://www.htmlwizard.net/resources/tutorials/regex_intro.html.
The POSIX Extended Regular Expression functions all (save split() and spliti() have names beginning with ereg.
Perl-Compatible Regular Expression FunctionsPerl-compatible regular expression (PCRE) functions bring Perl's parsing power to PHP. The syntax of the regular expression patterns is almost the same as Perl's, except for a few custom PHP-specific modifications. Every pattern should be enclosed by the delimiters. Any character can be used as a delimiter as long as it's not alphanumeric or a backslash. When the delimiter character has to be used in the pattern itself, it needs to be escaped by a backslash. As in Perl, the ending delimiter may be followed by optional modifiers that affect how the matching and pattern processing is done.
The Perl-compatible Regular Expression functions all have names beginning with preg.
Pattern ModifiersThe following list contains all possible modifiers supported by the PCRE functions in PHP. Some Perl-specific modifiers are not supported, and conversely, there are some modifiers that Perl doesn't have.
- i
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When this modifier is used, the matching of alphabetic characters in the pattern becomes non-case-sensitive; for example, "/sgi/i" matches both "sgi" and "SGI." This is equivalent to Perl's /i modifier.
- m
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By default, PCRE treats the subject string as consisting of a single "line" of characters (even if it actually contains several newlines). The "start of line" metacharacter (^) matches only at the start of the string, while the "end of line" metacharacter ($) matches only at the end of the string, or before a terminating newline (unless the D modifier is also set). This is the same as in Perl.
When this modifier is used, the "start of line" and "end of line" constructs match immediately following or immediately before any newline in the subject string, respectively, as well as at the very start and end. This is equivalent to Perl's /m modifier. If there are no "\n" characters in a subject string, or no occurrences of ^ or $ in a pattern, setting this modifier has no effect.
- s
-
When this modifier is used, a dot metacharacter (.) in the pattern matches all characters, including newlines. Without it, newlines are excluded. This modifier is equivalent to Perl's /s modifier. A negative class such as [^a] always matches a newline character, independent of the setting of this modifier.
- x
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When this modifier is used, whitespace data characters in the pattern are ignored except when escaped or inside a character class, and characters between an unescaped # outside a character class and the next newline character, inclusive, are also ignored. This is equivalent to Perl's /x modifier, and makes it possible to include comments inside complicated patterns. Note, however, that this applies only to data characters. Whitespace characters cannot appear within special character sequences in a pattern, for example within the sequence (?(, which introduces a conditional subpattern.
- e
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When this modifier is used, preg_replace() does normal substitution of references in the replacement string, evaluates it as PHP code, and uses the result of the evaluation for replacing the match found by the pattern.
Only preg_replace() uses this modifier; it's ignored by other PCRE functions.
- A
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When this modifier is used, the pattern is forced to be "anchored"; that is, it's constrained to match only at the start of the string that's being searched (the "subject string"). This effect can also be achieved by appropriate constructs in the pattern itself, which is the only way to do it in Perl.
- D
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When this modifier is used, a dollar metacharacter ($) in the pattern matches only at the end of the subject string. Without this modifier, a dollar sign also matches immediately before the final character if it's a newline (but not before any other newlines). This modifier is ignored if the /m modifier is set. There is no equivalent to this modifier in Perl.
- S
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When a pattern is going to be used several times, it's worth spending more time analyzing it in order to speed up the time taken for matching. When this modifier is used, this extra analysis is performed. At present, studying a pattern is useful only for non-anchored patterns that don't have a single fixed starting character. This is equivalent to the study() function in Perl.
- U
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This modifier inverts the "greediness" of the quantifiers so that they're not greedy by default, but become greedy if followed by "?". Greedy quantifiers attempt to match as much of the target string as they legally can. The only limit on this behavior is that the greediness of one quantifier cannot cause the following other quantifiers in the pattern to fail. This modifier is not compatible with Perl.
- X
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This modifier turns on additional functionality of PCRE that is incompatible with Perl. Any backslash in a pattern that's followed by a letter that has no special meaning causes an error, thus reserving these combinations for future expansion. By default, as in Perl, a backslash followed by a letter with no special meaning is treated as a literal. At present, no other features are controlled by this modifier.
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