Im not sure what this regex is called or how to write it

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

I guess this might be one of those look-ahead assertions or something, but frequently I'm seeking something like

scalar starts with "cat" but not ones followed anywhere with "dog".

I also have more complex conditions such as:

scalar starts with "cat" but not ones followed anywhere with "dog" except if its "dogs", or "dog" followed or preceeded by "collie".

I usually end up writing 2 or more regexs as cascading filters for these, and loop over the array- both undesirable since I'd prefer to use map(), and I think this could be one regex.

Wisdom please?

Comment on Im not sure what this regex is called or how to write it

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Re: Im not sure what this regex is called or how to write it by moritz (Cardinal) on Jul 28, 2011 at 13:54 UTC
scalar starts with "cat" but not ones followed anywhere with "dog". Yes, a negative look-ahead: `use strict; use warnings; for (qw/cat dog catfoo catdog/) { print $_ =~ /^cat(?!dog)/ ? '[X]' : '[ ]', "$_\n"; } __END__ [X]cat [ ]dog [X]catfoo [ ]catdog` [download] scalar starts with "cat" but not ones followed anywhere with "dog" except if its "dogs", or "dog" followed or preceeded by "collie". If the "dog" is proceeded by "collie", then it's not "cat followed by dog" anymore, so we don't have to consider that case. `use strict; use warnings; for (qw/cat dog catfoo catdog catdogs catdogcollie/) { print $_ =~ /^cat(?!dog(?!s\|collie))/ ? '[X]' : '[ ]', "$_\n"; } __END__ [X]cat [ ]dog [X]catfoo [ ]catdog [X]catdogs [X]catdogcollie` [download] Perl 6 - second systems done right	[reply] [d/l] [select]
Re^2: Im not sure what this regex is called or how to write it by misterperl (Friar) on Apr 20, 2012 at 14:34 UTC
Thank-You this was very helpful. Far moreso than see the documentation which of course could be an automated reply to ANY and ALL inquiries.	[reply]
Re: Im not sure what this regex is called or how to write it by jpl (Monk) on Jul 28, 2011 at 17:25 UTC
There's nothing `map` can do that cannot be accomplished by a loop. Unless you are after one-liner bragging rights, your cascading filters are likely to be a good deal easier to understand (and extend, and get right) than a single complicated regular expression. My advice would be to use the cascading filters unless there is a really compelling need not to.	[reply]
Re^2: Im not sure what this regex is called or how to write it by misterperl (Friar) on Dec 18, 2012 at 14:36 UTC
>> There's nothing map can do that cannot be accomplished by a loop. Unless you are after one-liner bragging rights.... Sadly, my first programming language I learned, c 1977, was APL, and henceforth I was conditioned to believe anything over 1 line was excess.. My Bad!	[reply]
Re^3: Im not sure what this regex is called or how to write it by AnomalousMonk (Archbishop) on Dec 18, 2012 at 15:30 UTC
... first programming language I learned ... conditioned* to believe ...* Another example of the "Rubber Duckie Syndrome."	[reply]
Re: Im not sure what this regex is called or how to write it by Anonymous Monk on Jul 28, 2011 at 13:50 UTC
What have you tried, what are your difficulties? That you don't know where to look in the documentation?	[reply]