$re = '/\((?<arguments>(([^\(\)\']*)|(\([^\(\)\']*\))|\'(.*?)\')*)\)/';
$str = 'Find arguments inside balanced parenthesis.
Regex idea from Marco https://stackoverflow.com/a/10596403/1898524
( arg1, arg2, arg3, (arg4), \'(pip\' )
trim(( arg1, arg2, arg2a, arg3, (arg4), \'(pip\' ))
SELECT ISNULL(string, left(1,13) )
SELECT ISNULL(string,\'x\' )
SELECT ISNULL(string, char(13) )
SELECT ISNULL(string, char(13) ), ISNULL(string, \'x\' )
ISNULL( x )
ISNULL( char(13))
SELECT MID(LTRIM(RTRIM(ISNULL(string, char(13)))),4,8) <-- Does not work on a heavily nested function.
';
preg_match_all($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