Re: Where can I find more examples of use of Special Backtracking Control Verbs?

I'd suggest Super Search. Which will basically show that AnomalousMonk is the local expert on the subject :P. After a quick browse, I have extracted two bits of interresting (to me) information. First (*SKIP) and (*FAIL) can be used for something that quacks like variable length look behind assertions, as discussed here. Second, (*FAIL) is identical to (?!), and one is actually compiled to the other.

I find this interesting because I have actually never used any of the control verbs - I remember having a vague understanding of what they do, and how to use them, but not why I would need them - while I have used look-around assertions (mostly look ahead) on several occasions and have a more intuitive understanding of them.

On the "variable length look behind assertion" case though, a far simpler solution has been made available with \K. This may be one reason why (*SKIP) and (*FAIL) aren't very common, but might also be another gateway to understanding what they do

Comment on Re: Where can I find more examples of use of Special Backtracking Control Verbs?

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Re^2: Where can I find more examples of use of Special Backtracking Control Verbs? by AnomalousMonk (Archbishop) on Nov 06, 2018 at 18:49 UTC
`\K` is the basis for a variable length positive look behind assertion. The case discussed here implements a variable length negative look behind assertion, which in Perl must always be emulated in some way (ATM). Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^3: Where can I find more examples of use of Special Backtracking Control Verbs? by Eily (Monsignor) on Nov 06, 2018 at 22:58 UTC
Ahem, yes. I failed to see that. Thanks pointing that out. On a completly far-fetched note, now that I think I understand the (SKIP)(FAIL) pattern, it seems to me that it only covers the subset of negative look behind assertions that do not come after any character has been matched. For example: `/^(.{5})+(?<!abc)d/` (which is the same as matching `/d(?!cba)(.{5})+$/` on the reverse string) can't be rewritten using the (SKIP)(FAIL) pattern (AFAICT). I'm sure this is a problem people stumble upon all the time :P	[reply] [d/l] [select]
Re^4: Where can I find more examples of use of Special Backtracking Control Verbs? by AnomalousMonk (Archbishop) on Nov 07, 2018 at 19:42 UTC
For example: `/^(.{5})+(?<!abc)d/` ... can't be rewritten using the (SKIP)(FAIL) pattern ...* I don't see how it could be done, but I'm still looking. Certainly the trick has its limits. Still, it's handy in a few situations. However, variable width negative look-behind is, perhaps fortunately, not something one tends to really need very often. :) Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]