Re: [Regexp] Finding failure point in RE

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