PHP ActiveX

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.

PHP ActiveX

<?php

$sbSubject = new COM("Chilkat.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 COM("Chilkat.JsonObject");
$json->EmitCompact = 0;

// 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) {
    print $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.
print 'numMatches: ' . $numMatches . "\n";
print $json->emit() . "\n";

?>