The following pattern matches a function foo() which may contain
balanced parentheses as the argument.

$re = qr{ (                    # paren group 1 (full function)
            foo
            (                  # paren group 2 (parens)
              \(
                (              # paren group 3 (contents of parens)
                (?:
                 (?> [^()]+ )  # Non-parens without backtracking
                |
                 (?2)          # Recurse to start of paren group 2
                )*
                )
              \)
            )
          )
        }x;


##</code><code>##

#!/bin/perl -w
use strict;
#(?:[^{}#]|(?:\\).)*
#(?: (?:\{2}+) (?:[^{}#]* | (?:\\.)) )*
my $re = qr'^ ( 
               { 
               (
                {                   
                (                     
                 (?:
                  (?> [^{}]+ )
                  |
                  (?2)
                 )*
                )
                }                     
               )*$
            )
          'x;
while (<>) {
  printf "%s\n", m{$re} ? "match" : "nomatch";
  /^q/ && exit;
}

##</code><code>##

my $re = qr'^ ( 
               {
               (?<R>
                {                   
                (                     
                 (?:
                  (?> [^{}]+ )
                  |
                  (\q<R>)
                 )*
                )
                }                     
               )*$
            )
          'x;