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PHP Regular Expression Functions


PHP Regular Expression Introduction

Regular expressions allow you to search for and replace patterns in strings.


Installation

The PHP regular expression functions are part of the PHP core. No installation is required to use these functions.


Runtime Configuration

These settings in php.ini can be used to limit the amount time or resources used when evaluating regular expressions.

Name Default Description Changeable
pcre.backtrack_limit "1000000" The maximum number of backtracks that the regular expression engine is allowed to do while evaluating an expression. (available since PHP 5.2) PHP_INI_ALL
pcre.recursion_limit "100000" The maximum recursion depth that the regular expression engine is permitted to reach while evaluating an expression. (available since PHP 5.2) PHP_INI_ALL
pcre.jit "1" When set to "1" this enables PCRE's (Perl-Compatible Regular Expressions) just-in-time compilation. (available since PHP 7.0) PHP_INI_ALL

PHP Regular Expression Functions

Function Description
preg_filter() Returns a string or an array with pattern matches replaced, but only if matches were found
preg_grep() Returns an array consisting only of elements from the input array which matched the pattern
preg_last_error() Returns an error code indicating the reason that the most recent regular expression call failed
preg_match() Finds the first match of a pattern in a string
preg_match_all() Finds all matches of a pattern in a string
preg_replace() Returns a string where matches of a pattern (or an array of patterns) are replaced with a substring (or an array of substrings) in a given string
preg_replace_callback() Given an expression and a callback, returns a string where all matches of the expression are replaced with the substring returned by the callback
preg_replace_callback_array() Given an array associating expressions with callbacks, returns a string where all matches of each expression are replaced with the substring returned by the callback
preg_split() Breaks a string into an array using matches of a regular expression as separators
preg_quote() Escapes characters that have a special meaning in regular expressions by putting a backslash in front of them

Regular Expression Modifiers

Modifiers can change how a search is performed.

Modifier Description
i Performs a case-insensitive search
m Performs a multiline search (patterns that search for the beginning or end of a string will match the beginning or end of each line)
u Enables correct matching of UTF-8 encoded patterns

Regular Expression Patterns

Brackets are used to find a range of characters:

Expression Description
[abc] Find one character from the options between the brackets
[^abc] Find any character NOT between the brackets
[0-9] Find one character from the range 0 to 9

Metacharacters

Metacharacters are characters with a special meaning:

Metacharacter Description
| Find a match for any one of the patterns separated by | as in: cat|dog|fish
. Find just one instance of any character
^ Finds a match as the beginning of a string as in: ^Hello
$ Finds a match at the end of the string as in: World$
\d Find a digit
\s Find a whitespace character
\b Find a match at the beginning of a word like this: \bWORD, or at the end of a word like this: WORD\b
\uxxxx Find the Unicode character specified by the hexadecimal number xxxx

Quantifiers

Quantifiers define quantities:

Quantifier Description
n+ Matches any string that contains at least one n
n* Matches any string that contains zero or more occurrences of n
n? Matches any string that contains zero or one occurrences of n
n{x} Matches any string that contains a sequence of X n's
n{x,y} Matches any string that contains a sequence of X to Y n's
n{x,} Matches any string that contains a sequence of at least X n's

Note: If your expression needs to search for one of the special characters you can use a backslash ( \ ) to escape them. For example, to search for one or more question marks you can use the following expression: $pattern = '/\?+/';