php - How to write a recursive regex that matches nested parentheses?

Question

I am trying to write a regexp which matches nested parentheses, e.g.:

"(((text(text))))(text()()text)(casual(characters(#$%^^&&#^%#@!&**&#^*!@#^**_)))"

A string like this should be matched, cause all the nested parentheses are closed, instead:

"(((text)))(text)(casualChars*#(!&#*(!))"

Should not, or better, should match at least the first "(((text)))(text)" part.

Actually, my regexp is:

 $regex = '/( (  (() ([^[]*?)  (?R)?  ())  ){0,}) /x';

But it does not work properly as I am expecting. How to fix that? Where am I wrong? Thanks!

Answer 1

This pattern works:

$pattern = '~ ( (?: [^()]+ | (?R) )*+ ) ~x';

The content inside parenthesis is simply describe:

"all that is not parenthesis OR recursion (= other parenthesis)" x 0 or more times

If you want to catch all substrings inside parenthesis, you must put this pattern inside a lookahead to obtain all overlapping results:

$pattern = '~(?= ( ( (?: [^()]+ | (?1) )*+ ) ) )~x';
preg_match_all($pattern, $subject, $matches);
print_r($matches[1]);

Note that I have added a capturing group and I have replaced (?R) by (?1):

(?R) -> refers to the whole pattern (You can write (?0) too)
(?1) -> refers to the first capturing group

What is this lookahead trick?

A subpattern inside a lookahead (or a lookbehind) doesn't match anything, it's only an assertion (a test). Thus, it allows to check the same substring several times.

If you display the whole pattern results (print_r($matches[0]);), you will see that all results are empty strings. The only way to obtain the substrings found by the subpattern inside the lookahead is to enclose the subpattern in a capturing group.

Note: the recursive subpattern can be improved like this:

( [^()]*+ (?: (?R) [^()]* )*+ )