Get nested bracket pairs from a string.

gistfile1.txt

<?php
/**
 * PHP: Algorithm for recursive string subset extraction
 * @link http://stackoverflow.com/q/14532139/367456
 * @link https://gist.github.com/4642090
 * @link http://eval.in/7578
 */

$subject = 'abc{def}ghij{kl{mn}o{pq}}r{s{t{u{v}}w}}xyz';

$pattern = '([}{])';
$offset = 0;
$stack = [];
$accept = function($subject) {
    echo $subject, "\n";
};

while (preg_match($pattern, $subject, $match, PREG_OFFSET_CAPTURE, $offset))
{
    list($token, $position) = $match[0];

    switch ($token) 
    {
        # TOKEN_OPEN
        case '{':
            $stack[] = $position;
            break;
        
        # TOKEN_CLOSE
        case '}':
            $start = array_pop($stack);
            $accept(substr($subject, $start + 1, $position - $start - 1));            
            break;

        default:
            throw new Exception(sprintf('Unexpected token "%s".', $token));
    }

    $offset = $position + 1;
}