PHP: unexpected PREG_BACKTRACK_LIMIT_ERROR

Question

<?

function recursiveSplit($string, $layer) {
    $err = preg_match_all("/\{(([^{}]*|(?R))*)\}/",$string,$matches);
    echo "Elementi trovati: $err<br>";
    if($err == FALSE) echo "preg_match_all ERROR<br>";

    // iterate thru matches and continue recursive split
    if (count($matches) > 1) {
        for ($i = 0; $i < count($matches[1]); $i++) {
            if (is_string($matches[1][$i])) {
                if (strlen($matches[1][$i]) > 0) {
                    echo "<pre>Layer ".$layer.":   ".$matches[1][$i]."</pre><br />";
                    recursiveSplit($matches[1][$i], $layer + 1);
                }
            }
        }
    }
}

$buffer = "{aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa{aaaaaaaaaaaaaaaaaa{aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa}";
recursiveSplit($buffer, 0);

output

Elementi trovati: 
preg_match_all ERROR
Backtrack limit was exhausted!

this code gives me a PREG_BACKTRACK_LIMIT_ERROR error... but the backtrack limit is set to 100.000.000.

Community · Accepted Answer · 2017-05-23 12:25:19Z

14

Another classic case of catastrophic backtracking. Must be my lucky day today.

/\{(([^{}]*|(?R))*)\}/

only matches if the braces are nested correctly. Which they aren't, of course, in your string.

Now the problem is that your regex needs to figure out all possible string combinations you can build with 106 as to figure that out because you have nested quantifiers ((...)*)*). Which (correct me if I'm wrong) should be somewhere in the vicinity of 106! which comes to

114628056373470835453434738414834942870388487424139673389282723476762012382449946252660360871841673476016298287096435143747350528228224302506311680000000000000000000000000

which easily beats your PREG_BACKTRACK_LIMIT.

If you use possessive quantifiers to make sure that you'll never backtrack into the non-braces you've already matched, then you should be OK:

/\{(([^{}]*+|(?R))*)\}/

edited May 23, 2017 at 12:25

CommunityBot

11 silver badge

answered Mar 13, 2012 at 20:56

Tim Pietzcker

337k59 gold badges521 silver badges572 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

stema Over a year ago

+1 Lucky you! An alternative to the possessive quantifier would be an atomic group /\{((?>[^{}]*|(?R))*)\}/ as it is used in the recursive example on php.net

Collectives™ on Stack Overflow

PHP: unexpected PREG_BACKTRACK_LIMIT_ERROR

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related