Remove the text outside the brackets

Question

i.e.

$text = 'remove this text (keep this text and 123)';

echo preg_replace('', '', $text);

It should output:

(keep this text and 123)

Answer 1

This will do it: (and works with nested () as well)

$re = '/[^()]*+(\((?:[^()]++|(?1))*\))[^()]*+/';
$text = preg_replace($re, '$1', $text);

Here are a couple test cases:

Input:
Non-nested case: 'remove1 (keep1) remove2 (keep2) remove3'
Nested case:     'remove1 ((keep1) keep2 (keep3)) remove2'

Output:
Non-nested case: '(keep1)(keep2)'
Nested case:     '(keep1) keep2 (keep3)'