Delphi ActiveX

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.

Delphi ActiveX

uses
    Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
    Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Chilkat_TLB;

...

procedure TForm1.Button1Click(Sender: TObject);
var
sbSubject: TChilkatStringBuilder;
i: Integer;
pattern: WideString;
json: TChilkatJsonObject;
numMatches: Integer;

begin
sbSubject := TChilkatStringBuilder.Create(Self);

// Create data that would cause a catastrophic backtrack with the regular expression "((a+)+$)"
i := 0;
while i < 500 do
  begin
    sbSubject.Append('aaaaaaaaaaaaaaaaaaaa');
    i := i + 1;
  end;

sbSubject.Append('X');

pattern := '((a+)+$)';

json := TChilkatJsonObject.Create(Self);
json.EmitCompact := 0;

// Set a time limit to prevent a catastrophic backtrack..
// (Approx) 1 second time limit.
// This should fail:
numMatches := sbSubject.RegexMatch(pattern,json.ControlInterface,1000);
if (numMatches < 1) then
  begin
    Memo1.Lines.Add(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

    Exit;
  end;

// We shouldn't get here.
// The above data and regular expression should've caused a catastrophic backtrack.
Memo1.Lines.Add('numMatches: ' + IntToStr(numMatches));
Memo1.Lines.Add(json.Emit());
end;