[FWP] innermost first parsing

[Jeff Pinyan <jeffp@crusoe.net>]

Given text like (indented for sanity's sake):

  BEGIN
    alpha
    BEGIN
      beta
      gamma
      BEGIN
        delta
      END
      epsiolon
      BEGIN
        zeta
        eta
      END
      theta
    END
  END

If you want to find the first piece of data between BEGIN and END that
does NOT have another BEGIN ... END set in it, you can use the following
"unrolling the loop" style regex:

  ($first) = $text =~ m{
    BEGIN
    (
      [^BE]*  # 'B' and 'E' are first chars of tags
      (?:
        (?:
          B+ (?! EGIN )  # match /B+/ if NOT 'BEGIN'
          |
          E+ (?! ND )    # match /E+/ if NOT 'END'
        )
        [^BE]*
      )*
    )
    END
  }xs;

This has worked for the stress-testing I've done.

The application is:

  while ($text =~ s/REGEX/SOMETHING/) {
    $count++;
  }

so that the inner-most matches are dealt with first.

-- 
Jeff "japhy" Pinyan     japhy@pobox.com     http://www.pobox.com/~japhy/
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