(Swift) Regular Expression Catastrophic Backtrack

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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.

Note: This example requires Chilkat v11.1.0 or greater.

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func chilkatTest() {
    let sbSubject = CkoStringBuilder()!

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

    sbSubject.append("X")

    var pattern: String? = "((a+)+$)"

    let json = CkoJsonObject()!
    json.emitCompact = false

    // Set a time limit to prevent a catastrophic backtrack..
    // (Approx) 1 second time limit.
    // This should fail:
    var numMatches: Int = sbSubject.regexMatch(pattern, json: json, timeoutMs: 1000).intValue
    if numMatches < 1 {
        print("\(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.
    print("numMatches: \(numMatches)")
    print("\(json.emit()!)")

}