Node.js

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.

Node.js

NODEJS_PRELUDE

function chilkatExample() {

    var sbSubject = new chilkat.StringBuilder();

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

    sbSubject.Append("X");

    var pattern = "((a+)+$)";

    var json = new chilkat.JsonObject();
    json.EmitCompact = false;

    //  Set a time limit to prevent a catastrophic backtrack..
    //  (Approx) 1 second time limit.
    //  This should fail:
    var numMatches = sbSubject.RegexMatch(pattern,json,1000);
    if (numMatches < 1) {
        console.log(sbSubject.LastErrorText);

        //  	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

        return;
    }

    //  We shouldn't get here.
    //  The above data and regular expression should've caused a catastrophic backtrack.
    console.log("numMatches: " + numMatches);
    console.log(json.Emit());

}

chilkatExample();