A Regex simple Question

pysome has asked for the wisdom of the Perl Monks concerning the following question:

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Re: A Regex simple Question by FunkyMonk (Bishop) on Sep 06, 2007 at 15:52 UTC
I'd split it into two matches: `while ( <DATA> ) { chomp; print "$_: ", /^a.b$/ && !/.ab./ ? "yes" : "no", "\n"; } __DATA__ a000000000b a0a0000b00b a0000ab000b ab00000000b a000ab` [download] Output: `a000000000b: yes a0a0000b00b: yes a0000ab000b: no ab00000000b: yes a000ab: yes` [download] update*: Added two more tests cases and fixed a bug	[reply] [d/l] [select]
Re: A Regex simple Question by Sidhekin (Priest) on Sep 06, 2007 at 15:54 UTC
Negative lookahead is always tricky ... you're on the right track though. In this specific case you could do this: `/^a(?!b)(?:(?!ab).)+b$/` [download] ... or even this: `/^(?!.ab)a.+b$/` [download] ... unless I've read you wrong, and you want to allow the string "ab" and even "ab000b"/"a000ab" (since the "ab" is not strictly between* the "a" and the "b"), in which case you want this: `/^a(?:(?!ab.).)b$/` [download] ... or even this: `/^(?!.+ab.)a.b$/` [download] Always tricky, the negative lookahead ... if you can split it, like FunkyMonk suggests (or some variation thereof), it'll be far more readable. `print "Just another Perl ${\(trickster and hacker)},"` The Sidhekin proves Sidhe did it!	[reply] [d/l] [select]
Re: A Regex simple Question by johngg (Canon) on Sep 06, 2007 at 15:55 UTC
A negative look-ahead is fine, just move the `.` into the assertion. `if ($s =~ /^a(?!.ab).b$/) {` [download] Cheers, JohnGG Update:* If you want `"a00000000ab"` to match then you could change the look-ahead to ensure there is at least one character after the `"ab"`. `use strict; use warnings; my @strings = qw{ a000000000b a0a0000b00b a0000ab000b a00000000ab }; print m{^a(?!.ab.).b$} ? qq{$_: matched\n} : qq{$_: unmatched\n} for @strings;` [download] which produces `a000000000b: matched a0a0000b00b: matched a0000ab000b: unmatched a00000000ab: matched` [download]	[reply] [d/l] [select]
Re: A Regex simple Question by ikegami (Patriarch) on Sep 06, 2007 at 15:54 UTC
Your have to make sure no position starts matches `/ab/`. `/^a(?:(?!ab).)b$/` [download] `(?:(?!regexp).)` is to regexps as `[^chars]` is to chars. Update: Not quite. `/$pre(?(?!$regexp).)$post/` only works when `/$regexp/` can't match something `/.$post/` matches. It doesn't work in this case because `/ab/` can match something that `/.b/` matches. It'll fail if the input is `a000ab`, for example. However, I suspect you have something closer to the following, which works fine: `/ <tag> # Start tag (?:(?!</tag>).)* # Body </tag> # End tag /x` [download]	[reply] [d/l] [select]
Re: A Regex simple Question by johnlawrence (Monk) on Sep 06, 2007 at 16:04 UTC
I don't like negative lookahead much either! My non-lookahead version: `/^a([^a]\|a[^b])*a?b$/` [download] Works as well I think.	[reply] [d/l]
Re: A Regex simple Question by moritz (Cardinal) on Sep 06, 2007 at 15:56 UTC
Your regex says "an a, followed by arbitrary characters, followed by arbitrary characters that don't start with ab, then a b". This means that the `(?!ab)` will always find a position where the next position is not ab. Knowing this, the solution is not so hard: `/^a(?!.ab).b$/` Perl 6 in German	[reply] [d/l] [select]
Re^2: A Regex simple Question by ikegami (Patriarch) on Sep 06, 2007 at 16:11 UTC
That will fail (false negative) for `a000ab`.	[reply] [d/l]
Re^3: A Regex simple Question by moritz (Cardinal) on Sep 06, 2007 at 16:36 UTC
You are right. That can easily be ammended by changing it to `/^a(?!.ab.).b$/`. Regexes are hard, sometimes. Especially corner cases ;) Perl 6 in German	[reply] [d/l]
Re: A Regex simple Question by injunjoel (Priest) on Sep 06, 2007 at 16:29 UTC
Update: Disregard this, I read too quickly initially. Just my 2 cents... `m/^a[^ab]*b$` [download] -InjunJoel "I do not feel obliged to believe that the same God who endowed us with sense, reason and intellect has intended us to forego their use." -Galileo	[reply] [d/l]