Go

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.

Go

    sbSubject := chilkat.NewStringBuilder()

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

    sbSubject.Append("X")

    pattern := "((a+)+$)"

    json := chilkat.NewJsonObject()
    json.SetEmitCompact(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 {
        fmt.Println(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

        sbSubject.DisposeStringBuilder()
        json.DisposeJsonObject()
        return
    }

    // We shouldn't get here.
    // The above data and regular expression should've caused a catastrophic backtrack.
    fmt.Println("numMatches: ", numMatches)
    fmt.Println(*json.Emit())

    sbSubject.DisposeStringBuilder()
    json.DisposeJsonObject()