PHP regular expression to replace nested () with []

Question

I am trying to match a string, see example, such that a nested parantheses () is replaced by [] so to not break a parser somewhere else. In this case, I would like to replace t

Answer 1

You could do this with a for loop and an array acting as a stack. When you find an open bracket, push onto the stack, and when you find a closing bracket pop from the stack. The length of the stack will tell you if you should replace the current bracket.

$stack = array();
$str = "Arman; Dario (10040 Druento (Turin), IT)";
$out = "";

for ($i = 0, $l = strlen($str); $i < $l; ++$i) {
    if ($str[$i] == "(") {
        $stack[] = true;

        if (count($stack) % 2) { // alternate between ( and [
            $out .= "(";
        } else {
            $out .= "[";
        }
    } else if ($str[$i] == ")") {
        if (count($stack) % 2) {
            $out .= ")";
        } else {
            $out .= "]";
        }
        array_pop($stack);
    } else {
        $out .= $str[$i];
    }
}

Here's some sample input and output:

Arman; Dario (10040 Druento (Turin), IT)
Arman; Dario (10040 Druento [Turin], IT)

a ( b ( c ( d ) ) e )
a ( b [ c ( d ) ] e )

a (b  (c) (d) (e) )
a (b  [c] [d] [e] )

a (b (c (d) (e) (f)))
a (b [c (d) (e) (f)])

It's not a particularly efficient algorithm (building the string char by char), and it could be a bit smarter about unmatched brackets, but anyway...