$re = '~\[if \s+ [^]]* ]
(?<content> [^[]*+ (?> (?R) [^[]* )*+ )
(?: \[elseif \s+ [^]]* ] \g<content> )*+
(?: \[else] \g<content> )?+
\[/if]~x';
$str = '$string = \'
[if true]
[if true]
[if false]
[else]
[/if]
[elseif false]
[if true]
[/if]
[else]
[/if]
[elseif false]
[/if]
[if false]
[if false]
[else]
[/if]
[else]
[if true]
[/if]
[/if]
\';';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php