You're right about the guess why "(" is necessary. Simply because the "?"
needs a precedence. It's a syntax thing.
"!" does not stand for "not". The combination "?!" stands for a "negative
look-ahead". See e.g. perldoc.perl.org/perlre.html for details.
Regards,
Boris
On Tue, July 9, 2013 16:56, Paul Gilmartin wrote:
> On Tue, 9 Jul 2013 11:34:55 +0200, Boris Lenz wrote:
>
>>does
>>
>>/(?!)/
>>
>>work for you?
>>
> Thanks! I never woulda thoughta that. Seems to work for sed and
> grep; nearly an exhaustive sample. Now I need to try to understand it:
>
> It matches any string which is not ("!") matched by 0 or 1 ("?") copies
> of the null string (which appears between "(" and "?"). The "(" and ")"
> indicate grouping. I have no idea why that's necessary. Perhaps
> operator precedence? Would any of /(?!)/, /(?)!/, or even /()?!/ work
> alike? Why not?
>
> And, of course, there are many flavors of regex, all with different rules.
>
> Thanks again,
> gil
>
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