php regex, get string in bracket

Assuming that you wish to match a parenthesized group which begins with PRN and which may contain nested parenthesized groups (and this whole chunk may itself be nested within enclosing parenthesized groups), then the following tested recursive regex solution will do the trick:

A recursive PCRE regex solution:

<?php // test.php 20140614_0800
// The regex:
$re = '/
    # Match nested parenthesized group beginning with PRN.
    \(PRN          # Literal opening sequence.
    (              # $1: Recursive subroutine!
      (?:          # Zero or more contents alternatives.
        [^()]++    # Either one or more non-parentheses,
      | \((?1)\)   # Or a nested parenthesized group.
      )*           # End zero or more contents alternatives.
    )              # End $1: Recursive subroutine!
    \)             # Literal closing sequence.
    /x';

// The string:
$s = '(ROOT  
(S
(NP (DT the) (NN author))
(VP (VBZ has)
  (VP (VBN failed)
    (S
      (VP (TO to)
        (VP
          (VP (VB catch)
            (PRT (RP up))
            (PP (IN with)
              (NP
                (NP (JJ recent) (NNS discoveries))
                (PP (IN about)
                  (NP (NNP Thespis))))))
          (PRN (, ,)
            (S
              (NP (NNP Gilbert)
                (CC and)
                (NNP Sullivan))
              (VP (VBZ \'s)
                (ADJP (JJ first))))
            (, ,))
          (NP (`` `) (JJ lost) (\'\' \') (JJ joint) (NN work)))))))
(. .)))';

// The code:
if (preg_match($re, $s, $matches)) {
    printf("Match found:\n%s", $matches[0]);
} else {
    echo('No match');
}
?>

Here is the output:

Match found:
(PRN (, ,)
            (S
              (NP (NNP Gilbert)
                (CC and)
                (NNP Sullivan))
              (VP (VBZ 's)
                (ADJP (JJ first))))
            (, ,))

Note that this solution requires that all the groups have properly balanced and matching open and close parentheses.

Are you just asking for an expression that will match the above output? If so, this works:

$output = "(ROOT
(S
(NP (DT the) (NN author))
(VP (VBZ has)
  (VP (VBN failed)
    (S
      (VP (TO to)
        (VP
          (VP (VB catch)
            (PRT (RP up))
            (PP (IN with)
              (NP
                (NP (JJ recent) (NNS discoveries))
                (PP (IN about)
                  (NP (NNP Thespis))))))
          (PRN (, ,)
            (S
              (NP (NNP Gilbert)
                (CC and)
                (NNP Sullivan))
              (VP (VBZ 's)
                (ADJP (JJ first))))
            (, ,))
          (NP (`` `) (JJ lost) ('' ') (JJ joint) (NN work)))))))
(. .)))";

preg_match('/\(PRN.*,\)\)/s', $output, $match);
print_r($match[0]);

Output:

php reg.php
(PRN (, ,)
            (S
              (NP (NNP Gilbert)
                (CC and)
                (NNP Sullivan))
              (VP (VBZ 's)
                (ADJP (JJ first))))
            (, ,))