Regular expression pattern explanation

$groups= qr/\(([^()]+|(??{$groups}))*\)/;

$groups = qr/
 \( # match an open paren (
 (# followed by
 [^()]+ # one or more non-paren character
 | # OR
 (??{$groups}) # the regex itself
 )* # repeated zero or more times
 \) # followed by a close paren )
 /x;

I am fairly new to regular expressions, I recently I used a regular expression in the perl script and I can’t figure it out:

$groups= qr/\(([^()]+|(? ?{$groups}))*\)/;

Any help would be greatly appreciated!

Well, if you extend it:

$groups= qr/
 \( # match an open paren (
 (# followed by
 [^()]+ # one or more non-paren character
 | # OR
 (?? {$groups}) # the regex itself
 )* # repeated zero or more times
 \) # followed by a close paren )
/x;

You will Get an elegant recursive method to find balanced brackets:)