Pockets in Replacement via Regular Expressions in PHP

When working with the function preg_replace, if we put something into a pocket in the regular expression, then in the replacement string we can insert the contents of this pocket by writing the dollar sign $ and the pocket number. For example, $0 - the zero pocket, $1 - the first pocket, $2 - the second pocket, and so on.

Let's look at examples to understand what this is for and how to use it.

Example

Let's find all numbers and replace them with the same numbers but in curly braces. To do this, all found numbers will be replaced by themselves, but in curly braces:

<?php
	$str = '1 23 456 xax';
	$res = preg_replace('#(\d+)#', '{$1}', $str);
?>

As a result, the following will be written to the variable:

'{1} {23} {456} xax'

Example

Let's find all strings representing numbers with x's around them and replace these numbers with themselves, but with '!' signs around:

<?php
	$str = 'x1x x23x x456x xax';
	$res = preg_replace('#x(\d+)x#', '!$1!', $str);
?>

As a result, the following will be written to the variable:

'!1! !23! !456! xax'

Example

Let's solve the following problem: given strings of the form 'aaa@bbb' - letters, then an at sign, then letters. We need to swap the letters before '@' and after. Let's implement it:

<?php
	$str = 'aaa@bbb ccc@ddd';
	$res = preg_replace('#([a-z]+)@([a-z]+)#', '$2@$1', $str);
?>

As a result, the following will be written to the variable:

'bbb@aaa ddd@ccc'

Practical Tasks

Given a string:

<?php
	$str = '12 34 56 78';
?>

Swap the digits in all two-digit numbers.

Given a string with a date:

<?php
	$str = '31.12.2025';
?>

Convert this date to '2025.12.31'.