Objective-C

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.

Objective-C

#import <CkoStringBuilder.h>
#import <NSString.h>
#import <CkoJsonObject.h>

CkoStringBuilder *sbSubject = [[CkoStringBuilder alloc] init];

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

[sbSubject Append: @"X"];

NSString *pattern = @"((a+)+$)";

CkoJsonObject *json = [[CkoJsonObject alloc] init];
json.EmitCompact = NO;

// Set a time limit to prevent a catastrophic backtrack..
// (Approx) 1 second time limit.
// This should fail:
int numMatches = [[sbSubject RegexMatch: pattern json: json timeoutMs: [NSNumber numberWithInt: 1000]] intValue];
if (numMatches < 1) {
    NSLog(@"%@",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.
NSLog(@"%@%d",@"numMatches: ",numMatches);
NSLog(@"%@",[json Emit]);