Re^2: Regex libraries

You´re right.
If I dont have an important feature which I would miss in the future its quite useless to whine about something I dont have and actually dont know how to apply correctly.
Perhaps I expressed myself unclear (sorry for that) I do have one or two cases where Lookaheads are usefull but I am still thinking I could also write them in Boost. ~~Unfortunatly I havent found out how yet~~:

Perl goes: /(?=.*\.)/

Boost goes: /[^.]*/

Actually I just got aware that the second regex without Lookahead is much more usefull because its more flexible. I simply didnt thought well enough about my problem.

I should take myself more time to think about problems. One week still isnt enough ;-) I probably should wipe my ideas if I havent solved them in one day.
The obvious often hides behind the complicated.

Anyhow. I just made the decision to use the library which is currently implemented. I was more frightend to have something I wouldnt be able to work with. And I needed to see this written down somewhere *g

Thanks for pointing me in the right direction

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Re^3: Regex libraries by ikegami (Patriarch) on Dec 31, 2004 at 18:58 UTC
`/(?=.\.)/` and `/[^.]/` are not the same thing at all. `'booga' =~ /[^.]/ matches. 'booga.' =~ /[^.]/ matches. 'booga' =~ /(?=.\.)/ does not match. 'booga.' =~ /(?=.\.)/ matches. '4.6.8' =~ /[^.]\.(\d)/ returns 6 in $1. '4.6.8' =~ /(?=.\.)(\d)/ returns 4 in $1. '4.6.8' =~ /((?=.\.))\1(\d)/ returns 8 in $2. . matched a "4.6" '4.6.8' =~ /.\.(\d)/ returns 8 in $1. Lookahead not needed. '4.6.8' =~ /(?:(?!\.).)(\d)/ returns 6 in $1. You want neg lookahead` [download] `/(?=.\.)/` requires a "." in the string. `/[^.]/` does not requires a "." in the string. `/[^.]/` is equivalent of the negative lookahead `/(?:(?!\.).)/`. The advantage of the negative lookahead version is that you can negatively match more than one character: `/(?:(?!$re).)/`. If I only wanted to negatively match one character, I'd use `[^...]`, even in Perl. Side thought: Hum, It would be nice if `/(?:(?!$re).)/` could be shortcutted to `/(?^$re)/`, since that's the typical use of a negative lookahead.	[reply] [d/l] [select]

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Re^3: Regex libraries
by ikegami (Patriarch) on Dec 31, 2004 at 18:58 UTC

/(?=.*\.)/ and /[^.]*/ are not the same thing at all.

'booga'  =~ /[^.]*/            matches.
'booga.' =~ /[^.]*/            matches.

'booga'  =~ /(?=.*\.)/         does not match.
'booga.' =~ /(?=.*\.)/         matches.


'4.6.8'  =~ /[^.]*\.(\d)/      returns 6 in $1.
'4.6.8'  =~ /(?=.*\.)(\d)/     returns 4 in $1.
'4.6.8'  =~ /((?=.*\.))\1(\d)/ returns 8 in $2. .* matched a "4.6"
'4.6.8'  =~ /.*\.(\d)/         returns 8 in $1. Lookahead not needed.
'4.6.8'  =~ /(?:(?!\.).)*(\d)/ returns 6 in $1. You want neg lookahead
[download]

/(?=.*\.)/ requires a "." in the string. /[^.]*/ does not requires a "." in the string. /[^.]*/ is equivalent of the negative lookahead /(?:(?!\.).)*/. The advantage of the negative lookahead version is that you can negatively match more than one character: /(?:(?!$re).)*/. If I only wanted to negatively match one character, I'd use [^...], even in Perl.

Side thought: Hum, It would be nice if /(?:(?!$re).)*/ could be shortcutted to /(?^$re)/, since that's the typical use of a negative lookahead.

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