Re: Re: parsing question

Abigail's extended regular expression is also an opportunity to show you a nifty module called YAPE::Regex::Explain.


ladoix% cat 290992
#!/usr/bin/perl

use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new(qr/_test(?>\s+)(?!<)/)->explain;


ladoix% perl 290992
The regular expression:

(?-imsx:_test(?>\s+)(?!<))

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  _test                    '_test'
----------------------------------------------------------------------
  (?>                      match (and do not backtrack afterwards):
----------------------------------------------------------------------
    \s+                      whitespace (\n, \r, \t, \f, and " ") (1
                             or more times (matching the most amount
                             possible))
----------------------------------------------------------------------
  )                        end of look-ahead
----------------------------------------------------------------------
  (?!                      look ahead to see if there is not:
----------------------------------------------------------------------
    <                        '<'
----------------------------------------------------------------------
  )                        end of look-ahead
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

--
Allolex

Comment on Re: Re: parsing question Download Code

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Re: parsing question by Abigail-II (Bishop) on Sep 12, 2003 at 12:50 UTC
Unfortunally, it only explains what it does, but it doesn't explain why it does so. Perhaps the most subtle part of the regex is `(?>\s+)`. Can you explain why it uses "no backtracking"? ;-) Abigail	[reply] [d/l]
Re: Re: parsing question by allolex (Curate) on Sep 12, 2003 at 13:06 UTC
I would love to know why you did what you did with that regex. There are probably a lot of monks in the Monastery who could learn something from you and just need a little push in the right direction. :) -- Allolex	[reply]
Re: parsing question by Abigail-II (Bishop) on Sep 12, 2003 at 13:13 UTC
`#!/usr/bin/perl use strict; use warnings; while (<DATA>) { chomp; print "with (?>): '$_' matches\n" if /_test(?>\s+)(?!<)/; print "without (?>): '$_' matches\n" if /_test\s+(?!<)/; } __DATA__ _test foo _test < _test < _test<` [download] Running this gives: `with (?>): '_test foo' matches without (?>): '_test foo' matches without (?>): '_test <' matches` [download] Abigail	[reply] [d/l] [select]
2Re: parsing question by bart (Canon) on Sep 13, 2003 at 10:31 UTC
I'm sure you're familiar with the "cut" operator, Abigail, but most of the other people here will find it largely underdocumented. So I dare to feel free to point towards the draft of a book on regular expressions that the same perlmonk is writing, who also wrote YAPE::Regex::Explain (actually, he wrote the whole YAPE suite). I personally like the draft of book very much, and so, I like to plug it whenever I can. So here's the URL: http://japhy.perlmonk.org/book/. Check out chapter 8 for the "cut" operator — every chapter is a separate download, roughly around 12 pages each, either in MS Word or in PDF format. Recommended.	[reply]