$re = '/{if ([^}]+)}([\W\w]*?)(?:{else}([\W\w]*?))?{\/if}/';
$str = '{if $module=admin}
{/if}
{if $module=admin}
blah{invalid} blah
{else}
sdf
{/if}';
preg_match_all($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