You're thinking too hard. I cheated. My patch just fools the regex engine into thinking it hasn't actually started matching yet. Here's a drawn-out example.

$str = "abc.def.ghi.jkl";
$str =~ s{
  .*     # match as much as you can
  \K     # and then pretend HERE is where we start
  \. .*  # then a . and anything else
}{}x;    # replace with nothing

__END__
abc.def.ghi.jkl       $&
AAAAAAAAAAA     .*    "abc.def.ghi"
                \K    ""
           B    \.    "."
           BCCC .*    ".jkl"
[download]

Does that help you see what I do? My patch consists of a couple lines of support, but this is the beef:

case KEEP:
  PL_regstartp[0] = locinput - PL_bostr;
  break;
[download]

That's what happens when the regex engine encounters the \K. The rest of the patch is just creating the "KEEP" node, and telling toke.c that "\K" is a valid escape sequence.

_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area)
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;