PHP Requires Chilkat v11.1.0+

Regular Expression Catastrophic Backtrack

See more Regular Expressions Examples

This example demonstrates how adding a processing time limit prevents a catastrophic backtrack.

Catastrophic backtracking in regular expressions occurs when a poorly constructed pattern causes the regex engine to try an exponential number of possibilities, especially on non-matching input. This leads to extremely slow performance or even a program hang.

Example:

(a+)+$

Applied to:

aaaaaaaaaaaaaaaaaaaaaab

The regex engine tries many combinations of grouping a+ inside another +, looking for a way to match the whole string, but it never matches due to the final b. The nested quantifiers (+ inside +) are what trigger the backtracking explosion.

How to prevent it:

• Avoid nested quantifiers like (a+)+
• Use atomic groups or possessive quantifiers if available
• Consider more efficient regex design or a parser

Catastrophic backtracking is especially dangerous when regex patterns are applied to user-controlled input.

Chilkat2 PHP

<?php

use Chilkat2\JsonObject;
use Chilkat2\StringBuilder;

$sbSubject = new StringBuilder();

// Create data that would cause a catastrophic backtrack with the regular expression "((a+)+$)"
$i = 0;
while ($i < 500) {
    $sbSubject->Append('aaaaaaaaaaaaaaaaaaaa');
    $i = $i + 1;
}

$sbSubject->Append('X');

$pattern = '((a+)+$)';

$json = new JsonObject();
$json->EmitCompact = false;

// Set a time limit to prevent a catastrophic backtrack..
// (Approx) 1 second time limit.
// This should fail:
$numMatches = $sbSubject->RegexMatch($pattern, $json, 1000);
if ($numMatches < 1) {
    echo $sbSubject->LastErrorText, "\n";

    //     We should get an error such as the following:

    //     ChilkatLog:
    //       RegexMatch:
    //         ChilkatVersion: 11.1.0
    //         regex_match:
    //           timeoutMs: 1000
    //           Exceeded regular expression match limit.
    //           elapsedMs: Elapsed time: 797 millisec
    //           num_matches: -1
    //         --regex_match
    //       --RegexMatch
    //     --ChilkatLog

    exit;
}

// We shouldn't get here.
// The above data and regular expression should've caused a catastrophic backtrack.
echo 'numMatches: ', $numMatches, "\n";
echo $json->Emit(), "\n";