In RFC 72, Peter Heslin gives this example:
:Imagine a very long input string containing data such as this:
:
: ... GCAAGAATTGAACTGTAG ...
:
:If you want to match text that matches /GA+C/, but not when it
:follows /G+A+T+/, you cannot at present do so easily.
I haven't tried to work it out exactly, but I think you can
achieve this (and fairly efficiently) with something like:
/
(?: ^ | # else we won't match at start
(?: (?> G+ A+ T+) | (.) )*
(?(1) | . )
)
G A+ C
/x
This requires that the regexp engine reliably leaves $1 unset if
we took the G+A+T+ branch last time through the (...)*, which
has been an area of many bugs and no little discussion in perl5;
I'm not sure of the status of that in latest perls.
It isn't particularly relevant to this proposal since there are
other combinations that can't be resolved in this way; I thought
it might be of interest nonetheless.
Hugo
