Mastering PHP's preg_replace: Syntax, Parameters, and Real-World Examples

Function Overview

preg_replace() performs a regular‑expression search and replace on a string or an array of strings.

How It Works

The function scans the $subject for matches to $pattern and substitutes each match with $replacement. If $subject is an array, the operation is applied to every element and an array is returned.

Parameter Details

pattern – The regex pattern to search for. It can be a string or an array of strings. PCRE modifiers, including the deprecated e, may be used.

replacement – The replacement string or array. When pattern is an array and replacement is a single string, that string is used for all patterns. If both are arrays, each pattern is paired with the corresponding replacement element; missing replacements are treated as empty strings.

subject – The input string or array of strings.

limit – Maximum number of replacements per pattern per subject. Default is -1 (no limit).

count – Optional variable that will be filled with the total number of replacements performed.

Return Value

If $subject is an array, an array of results is returned; otherwise a string is returned. If no matches are found, the original subject is returned unchanged. On error, NULL is returned.

Example 1 – Reformatting a Date

<?php
$string = 'April 15, 2020';
$pattern = '/(\w+) (\d+), (\d+)/i';
$replacement = '${1}1,$3';
echo preg_replace($pattern, $replacement, $string);
?>

Output:

April1,2020

Example 2 – Multiple Patterns and Replacements

<?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 – Using Count and Limit

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

Output:

xp***to
3