Technical FAQ
PHP Manual
CSS2 Manual
HTML Manual
JS Guide
JS Reference
PhpDock Manual
Nu-Coder Manual
PhpExpress Manual
PHP Joomla
Learn PHP
Last updated: Tue, 19 Sep 2006


(PHP 3 >= 3.0.9, PHP 4, PHP 5)

preg_replace -- Perform a regular expression search and replace


mixed preg_replace ( mixed pattern, mixed replacement, mixed subject [, int limit [, int &count]] )

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

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.

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.

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.

The e modifier makes preg_replace() treat the replacement parameter as PHP code after the appropriate references substitution is done. Tip: make sure that replacement constitutes a valid PHP code string, otherwise PHP will complain about a parse error at the line containing preg_replace().



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


The string or an array with strings to replace. If this parameter is a string and the pattern parameter is an array, all pattens 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 less keys in the replacement array than in the pattern array, the excedent patterns will be replaced by an empty string.


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


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


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.


4.0.1pl2 Added the limit parameter
4.0.4 Added the '$n' form for the replacement parameter
5.1.0 Added the count parameter


Example 1. Using backreferences followed by numeric literals

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

The above example will output:


Example 2. Using indexed arrays with preg_replace()

$string = 'The quick brown fox jumped over the lazy dog.';
$patterns[0] = '/quick/';
$patterns[1] = '/brown/';
$patterns[2] = '/fox/';
$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.

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

$patterns = array ('/(19|20)(\d{2})-(\d{1,2})-(\d{1,2})/',
$replace = array ('\3/\4/\1\2', '$\1 =');
echo preg_replace($patterns, $replace, '{startDate} = 1999-5-27');

The above example will output:

$startDate = 5/27/1999

Example 4. Using the 'e' modifier


This would capitalize all HTML tags in the input text.

Example 5. Strip whitespace

This example strips excess whitespace from a string.

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

Example 6. Using the count parameter

$count = 0;

echo preg_replace(array('/\d/', '/\s/'), '*', 'xp 4 to', -1 , $count);
echo $count; //3

The above example will output:



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().

Last updated: Tue, 19 Sep 2006