Re: [Regexp] Finding failure point in RE

[Wes Biggs]

mitch-GNU RegExp List wrote:

> I posted this question a while but got no response, so I'll try once more...
>
> Is there a way (or plans to develop a way) to discover where in a regular
> expression that matching failed (i.e. didn't find a match)?  Or alternately,
> but not as useful, where in the regular expression is the last point that
> successfully matched the input string?
>
> Background:  I created a system that uses regular expressions to match
> against the contents of incoming emails that contain output from various
> status checks, operational tasks, etc.  When a match fails, it is a time
> consuming processes to discover where the failure point is.  A index into
> the regular expression (or input string I guess) that showed where the match
> failed would be very useful and time saving.
>  

Hi Mitch -- the short answer is no, there is not currently a way or a 
plan to implement this.

I'm assuming you're applying this to a situation where you're using 
isMatch() -- otherwise the logic gets a little ambiguous, because a 
failed RE will fail at every point along the input.

You could add a method like
int RE::getLengthMatched(input)
which would execute similarly to isMatch() but keep the contextual 
information such that
RE.isMatch(input) ==> (RE.getLengthMatched(input) == input.length())

Here's some untested off-the-cuff code you can try adding to RE.java:

public int getLengthMatched(Object o, int index, int eflags) {
   CharIndexed input = makeCharIndexed(o, index);
   if (firstToken == null)  { return 0; } // Trivial case of empty regexp
   REMatch m = new REMatch(numSubs, index, eflags);
   if (firstToken.match(input, m)) {
    int max = 0;
   while (m != null) {
       if (m.index > max) { max = m.index; }
       m = m.next;
   }
   }
   return max;
 }

Wes