Regex for checking if a string has mismatched parentheses?

Question

In a PHP script, what regex should I use to check for mismatched parentheses in a string? Things that I want to allow include:

• This is (ok)
• This (is)

Answer 1

Working php without regex:

function analyse($input){
    $len = strlen($input);
    $depth = 0;
    for ($i = 0; $i < $len; $i++) {
        $depth += $input[$i] == '(';
        $depth -= $input[$i] == ')';
        if ($depth < 0) break;
    }
    if ($depth != 0) return false;
        else return true;
}
$check_nestled = analyse('(5 * 2) + ((2 + 2) - 4)');
if($check_nestled){
    // do stuff, everything is ok
}