Understanding PHP preg_replace: Syntax, Parameters, and Practical Examples

preg_replace() executes a regular expression search and replace in PHP.

It searches the $subject for matches to $pattern and replaces them with $replacement. Optional parameters $limit and &$count control the maximum replacements and capture the number of replacements performed.

Parameters pattern: the regex pattern or an array of patterns; PCRE modifiers (including the deprecated e) may be used. replacement: the replacement string or array; can contain back‑references such as \$1 or \\1, with special handling for adjacent digits (e.g., ${1}1). subject: the string or array to be processed; if an array, the function returns an array of results. limit: maximum number of replacements per pattern (default -1 for unlimited). count: variable passed by reference that receives the total number of replacements.

Return value : the modified string, or an array when subject is an array; NULL is returned on error.

Example 1

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

Output: April1,2003 Example 2

<?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);
?>

Output: The bear black slow jumped over the lazy dog. Example 3

<?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');
?>

Output: $startDate=5/27/1999 Example 4

<?php
$str = 'foo   o';
$str = preg_replace('/\s\s+/', ' ', $str);
// Resulting string is 'foo o'
echo $str;
?>

Output: foo o Example 5

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

Output: *** to and the replacement count 3.