Author: ph10 Date: To: ND CC: Pcre-dev Subject: Re: [pcre-dev] No capture in nested negative assertions
On Tue, 10 Jul 2018, ND via Pcre-dev wrote:
> I guess Perl doesn't backtrack if last alternative of negative assertion
> fails.
Not quite (see below). I suspect it is just the way the Perl code turned
out rather than a deliberate design. I couldn't find any Perl
documentation about this.
> PCRE2 version 10.31 2018-02-12
> /(?!(a)b)/allcaptures
> a
> 0:
> 1: <unset>
Indeed, but the way Perl works means that adding another branch changes
things:
Perl 5.026002 Regular Expressions
/(?!(a)b)/
a
0:
1: a
/(?!(a)b|ac)/
a
0:
/(?!ac|(a)b)/
a
0:
It seems to save the capture only if there is just one branch in the
assertion. Or maybe it has some algorithm for deciding on which branch
to try first ... I don't know, but it all seems very inconsistent to me.
PCRE2 consistently discards the captures in all cases.
