Lianja

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.

Lianja

loSbSubject = createobject("CkStringBuilder")

// Create data that would cause a catastrophic backtrack with the regular expression "((a+)+$)"
i = 0
do while i < 500
    loSbSubject.Append("aaaaaaaaaaaaaaaaaaaa")
    i = i + 1
enddo
loSbSubject.Append("X")

lcPattern = "((a+)+$)"

loJson = createobject("CkJsonObject")
loJson.EmitCompact = .F.

// Set a time limit to prevent a catastrophic backtrack..
// (Approx) 1 second time limit.
// This should fail:
lnNumMatches = loSbSubject.RegexMatch(lcPattern,loJson,1000)
if (lnNumMatches < 1) then
    ? loSbSubject.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

    release loSbSubject
    release loJson
    return
endif

// We shouldn't get here.
// The above data and regular expression should've caused a catastrophic backtrack.
? "numMatches: " + str(lnNumMatches)
? loJson.Emit()


release loSbSubject
release loJson