Negative Lookahead Assertion Problem

Limbic~Region has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: Negative Lookahead Assertion Problem by itub (Priest) on Jul 18, 2005 at 23:13 UTC
Let's say you are trying to match `/\w+::\w+(?!\s\()/` against 'Foo::Bar ('. Initially the `\w+::\w+` will match "Foo::Bar"; then it notices that the assertion fails (because what follows is " ("), so it will backtrack to Foo::Ba, where it will try the assertion again, and it will succeed (because "r (" isn't matched by `\s(` ). To get what you want, you could try to ensure that you reached the end of the word, with something like `/\w+::\w+\b(?!\s*\()/` .	[reply] [d/l] [select]
Re^2: Negative Lookahead Assertion Problem by Anonymous Monk on Jul 18, 2005 at 23:34 UTC
/\w+::(?>\w+)(?!\s*\()/ might be better for the person that has to read your code later	[reply]
Re: Negative Lookahead Assertion Problem by Enlil (Parson) on Jul 18, 2005 at 23:40 UTC
Since your question as to why it doesn't work has been answered. (Basically it will backtrack till it does, and by moving a character back satisfies the regex.) This should work:`/\w+::(?>\w+)(?!\s\()/` `#!/usr/bin/perl use strict; use warnings; my @strings = ( '1. Foo::Bar', '2. Foo::Bar(', '3. Foo::Bar (', '4. Foo::Bar ' ); for ( @strings ) { print $_,$/ if /\w+::(?>\w+)(?!\s\()/; } __END__ 1. Foo::Bar 4. Foo::Bar` [download] Have a look at perlre where it reads about `(?>pattern)` -Enlil	[reply] [d/l] [select]
Re: Negative Lookahead Assertion Problem by BrowserUk (Patriarch) on Jul 18, 2005 at 23:41 UTC
How about `/Foo::Bar(?!\s\()(?=\W\|$)/ #or /\w+::\w+(?!\s\()(?=\W\|$)/` [download] Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal? "Science is about questioning the status quo. Questioning authority". The "good enough" maybe good enough for the now, and perfection maybe unobtainable, but that should not preclude us from striving for perfection, when time, circumstance or desire allow.	[reply] [d/l]
Re: Negative Lookahead Assertion Problem by monarch (Priest) on Jul 18, 2005 at 23:58 UTC
Surely trying to match `\s` is futile in a negative-lookup? Because `\s` can match nothing it will always succeed, thereby never matching?? I tried using `\s+`. The following code: `#!/usr/bin/perl -w use strict; my @strings = ( "Foo::Bar()", "Foo::Bar ()", "Foo::Bar ()", " Foo::Bar()", " Foo::Bar ()" ); my %test = ( 1 => qr/Foo::Bar(?!\s*\()/, 2 => qr/Foo::Bar(?!\s+\()/ );` [download] Read more... (590 Bytes) produces the following output `---Foo::Bar()--- test 1 does not match test 2 matches ---Foo::Bar ()--- test 1 does not match test 2 does not match ---Foo::Bar ()--- test 1 does not match test 2 does not match --- Foo::Bar()--- test 1 does not match test 2 matches --- Foo::Bar ()--- test 1 does not match test 2 does not match` [download]	[reply] [d/l] [select]
Re^2: Negative Lookahead Assertion Problem by ysth (Canon) on Jul 19, 2005 at 06:25 UTC
You missed the literal ( after the \s. Together, \s( does specify a meaningful assertion. In your tests, all your sample data is supposed to not match by my understanding of the OP. Only something such as "Foo::Bar is my favorite module" should match.	[reply]
Re^3: Negative Lookahead Assertion Problem by monarch (Priest) on Jul 19, 2005 at 11:39 UTC
I'm confused by your post. Breaking down `qr/Foo::Bar(?!\s+\()/` we get: `Foo::Bar # literal 'Foo::Bar' (?! # start of negative look-ahead \s+ # one or more white space chars \( # literal opening bracket ) # end of negative look-ahead` [download] so to answer your reply, I believe I provided the literal ( after the \s*. To answer your other statement, that something such as "Foo::Bar is my favourite module" should match, I agree - that is how I read the initial requirement from the thread. In which case my answer of using `\s+` kept within the spirit of the original statements made by the thread author. However I wanted to keep my example in line with the example that the author had tried, hence the use of the literal (. Hope this clears up my answer.	[reply] [d/l] [select]
Re^4: Negative Lookahead Assertion Problem by ysth (Canon) on Jul 19, 2005 at 20:12 UTC
Re^2: Negative Lookahead Assertion Problem by Limbic~Region (Chancellor) on Jul 19, 2005 at 14:07 UTC
monarch, I have to agree with ysth. The \s* is not meaningless because it is followed immediately by a literal (which is what his comment was referring to). To be a complete match it has to match 0 or more whitespaces and an open paren. In your solution there has to be at least 1 space - which doesn't fit the bill. Cheers - L~R	[reply]
Re: Negative Lookahead Assertion Problem by Anonymous Monk on Jul 18, 2005 at 23:19 UTC
someone on IRC pointed out: /(\w+?::\w+)\b(?!\s*\()/ It's not the answer to your question, though	[reply]
Re^2: Negative Lookahead Assertion Problem by Anonymous Monk on Jul 18, 2005 at 23:32 UTC
you have an extra ?	[reply]
Re: Negative Lookahead Assertion Problem (Perl 6) by kelan (Deacon) on Jul 19, 2005 at 12:10 UTC
Here's how to do it in Perl 6 (I believe): `rx/ \w+ \:\: \w+ :: <!before \s*(> / # # ^^ important` [download] This is basically your example with a small addition. The double colons pointed out above tell the engine not to backtrack once it reaches that point, so it will work this way where it wouldn't in Perl 5.	[reply] [d/l]