preg_replace_callback
preg_split
PCRE Functions
PHP Manual

# preg_replace

(PHP 4, PHP 5)

preg_replace — Perform a regular expression search and replace

### Description

mixed preg_replace ( mixed `$pattern` , mixed `$replacement` , mixed `$subject` [, int `$limit` = -1 [, int `&$count` ]] )

Searches `subject` for matches to `pattern` and replaces them with `replacement`.

### Parameters

`pattern`

  The pattern to search for. It can be either a string or an array with strings.

  Several PCRE modifiers are also available, including the deprecated 'e' (PREG_REPLACE_EVAL), which is specific to this function.

`replacement`

  The string or an array with strings to replace. If this parameter is a string and the `pattern` parameter is an array, all patterns will be replaced by that string. If both `pattern` and `replacement` parameters are arrays, each `pattern` will be replaced by the `replacement` counterpart. If there are fewer elements in the `replacement` array than in the `pattern` array, any extra `pattern`s will be replaced by an empty string.

  `replacement` may contain references of the form \\n or (since PHP 4.0.4) $n, with the latter form being the preferred one. Every such reference will be replaced by the text captured by the n'th parenthesized pattern. n can be from 0 to 99, and \\0 or $0 refers to the text matched by the whole pattern. Opening parentheses are counted from left to right (starting from 1) to obtain the number of the capturing subpattern. To use backslash in replacement, it must be doubled ("\\\\" PHP string).

  When working with a replacement pattern where a backreference is immediately followed by another number (i.e.: placing a literal number immediately after a matched pattern), you cannot use the familiar \\1 notation for your backreference. \\11, for example, would confuse preg_replace() since it does not know whether you want the \\1 backreference followed by a literal 1, or the \\11 backreference followed by nothing. In this case the solution is to use \${1}1. This creates an isolated $1 backreference, leaving the 1 as a literal.

  When using the deprecated e modifier, this function escapes some characters (namely ', ", \ and NULL) in the strings that replace the backreferences. This is done to ensure that no syntax errors arise from backreference usage with either single or double quotes (e.g. 'strlen(\'$1\')+strlen("$2")'). Make sure you are aware of PHP's string syntax to know exactly how the interpreted string will look.

`subject`

  The string or an array with strings to search and replace.

  If `subject` is an array, then the search and replace is performed on every entry of `subject`, and the return value is an array as well.

`limit`

  The maximum possible replacements for each pattern in each `subject` string. Defaults to -1 (no limit).

`count`

  If specified, this variable will be filled with the number of replacements done.

### Return Values

preg_replace() returns an array if the `subject` parameter is an array, or a string otherwise.

If matches are found, the new `subject` will be returned, otherwise `subject` will be returned unchanged or `NULL` if an error occurred.

### Errors/Exceptions

An `E_DEPRECATED` level error is emitted when passing in the "\e" modifier.

### Changelog

Version | Description
--- | ---
5.5.0 | The /e modifier is deprecated. Use preg_replace_callback() instead. See the PREG_REPLACE_EVAL documentation for additional information about security risks.
5.1.0 | Added the `count` parameter
4.0.4 | Added the '$n' form for the `replacement` parameter
4.0.2 | Added the `limit` parameter

### Examples

Example #1 Using backreferences followed by numeric literals

`<?php
$string = 'April 15, 2003';
$pattern = '/(\w+) (\d+), (\d+)/i';
$replacement = '${1}1,$3';
echo preg_replace($pattern, $replacement, $string);
?>`

The above example will output:

    April1,2003

Example #2 Using indexed arrays with preg_replace()

`<?php
$string = 'The quick brown fox jumped over the lazy dog.';
$patterns = array();
$patterns[0] = '/quick/';
$patterns[1] = '/brown/';
$patterns[2] = '/fox/';
$replacements = array();
$replacements[2] = 'bear';
$replacements[1] = 'black';
$replacements[0] = 'slow';
echo preg_replace($patterns, $replacements, $string);
?>`

The above example will output:

    The bear black slow jumped over the lazy dog.

By ksorting patterns and replacements, we should get what we wanted.

`<?php
ksort($patterns);
ksort($replacements);
echo preg_replace($patterns, $replacements, $string);
?>`

The above example will output:

    The slow black bear jumped over the lazy dog.

Example #3 Replacing several values

`<?php
$patterns = array ('/(19|20)(\d{2})-(\d{1,2})-(\d{1,2})/',
'/^\s*{(\w+)}\s*=/');
$replace = array ('\3/\4/\1\2', '$\1 =');
echo preg_replace($patterns, $replace, '{startDate} = 1999-5-27');
?>`

The above example will output:

    $startDate = 5/27/1999

Example #4 Strip whitespace

This example strips excess whitespace from a string.

`<?php
$str = 'foo o';
$str = preg_replace('/\s\s+/', ' ', $str);
// This will be 'foo o' now
echo $str;
?>`

Example #5 Using the `count` parameter

`<?php
$count = 0;
echo preg_replace(array('/\d/', '/\s/'), '*', 'xp 4 to', -1 , $count);
echo $count; //3
?>`

The above example will output:

    xp***to
    3

### Notes

> Note:

> When using arrays with `pattern` and `replacement`, the keys are processed in the order they appear in the array. This is not necessarily the same as the numerical index order. If you use indexes to identify which `pattern` should be replaced by which `replacement`, you should perform a ksort() on each array prior to calling preg_replace().

### See Also

- PCRE Patterns
- preg_filter() - Perform a regular expression search and replace
- preg_match() - Perform a regular expression match
- preg_replace_callback() - Perform a regular expression search and replace using a callback
- preg_split() - Split string by a regular expression
- preg_last_error() - Returns the error code of the last PCRE regex execution

preg_replace_callback
preg_split
PCRE Functions
PHP Manual