PHP Manual
PCRE Functions
Perform a regular expression search and replace

preg_replace

(PHP 4, PHP 5, PHP 7, PHP 8)

preg_replace — Perform a regular expression search and replace

Description

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.

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 patterns will be replaced by an empty string.

replacement may contain references of the form \n or $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. Note that backslashes in string literals may require to be escaped.

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. 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.

If the subject array is associative, keys will be preserved in the returned value.

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

Using the "\e" modifier is an error; an E_WARNING is emitted in this case.

If the regex pattern passed does not compile to a valid regex, an E_WARNING is emitted.

Examples

Example #1 Using backreferences followed by numeric literals


<?php
$string = 'April 15, 2003';
$pattern = '/(\w+) (\d+), (\d+)/i';
$replacement = '$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 jumps 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 jumps 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 jumps over the lazy dog.

Example #3 Replacing several values


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

Note:
When both pattern and replacement are arrays, matching rules will operate sequentially. That is, the second pattern/replacement pair will operate on the string that results from the first pattern/replacement pair, not the original string. If you want to simulate replacements operating in parallel, such as swapping two values, replace one pattern by an intermediary placeholder, then in a later pair replace that intermediary placeholder with the desired replacement.

<?php $p = array('/a/', '/b/', '/c/'); $r = array('b', 'c', 'd'); print_r(preg_replace($p, $r, 'a')); // prints d ?>