A Regular Expression Bug

Several years ago, I specialized in Delphi software development.

Amongst several other endeavors, I wrote and sold a Regular Expression matcher. Well, it turns out that my matcher has a significant bug that was only just noticed by a user - handling of “$” to require a match to finish at the end of the line was broken.

Yikes!

For future reference of anyone consulting with Professor Google or Doctor Bing, here’s the fix.

The code inside TniRegularExpressionMatcher.MatchExpression is missing a check for mfFinishOnly.

Inside the repeat loop:

if oState.Accept then begin
  MatchFound(...);
  Result := true;
  if not bContinue then
    break;
end;

This code needs to check for mfFinishOnly as follows:

if oState.Accept then begin
  if not (mfFinishOnly in FxFlags)
     or (iScan=Length(aString)) then
  begin
    MatchFound(...);
    Result := true;
    if not bContinue then
      break;
  end;
end;

In english: If mfFinishOnly is not specified, any match is acceptable; if we’re at the end of the string, the match is always acceptable.