Re: [Emacs-diffs] master 938d252 4/4: Make regex matching reentrant; update syntax during match

[Daniel Colascione]

On 06/19/2018 07:30 AM, Perry E. Metzger wrote:
> On Tue, 19 Jun 2018 06:49:03 -0700 Daniel Colascione
> <address@hidden> wrote:
> > On 06/19/2018 06:44 AM, Perry E. Metzger wrote:
> > > On Tue, 19 Jun 2018 09:40:05 -0400 "Perry E. Metzger"
> > > <address@hidden> wrote:
> > > > On Mon, 18 Jun 2018 10:49:46 -0700 "Daniel Colascione"
> > > > <address@hidden> wrote:
> > > > > > I think the right thing would be to use TRE if we could get
> > > > > > Ville Laurikari to let someone else be the maintainer. Sadly
> > > > > > right now it seems unmaintained and has been for years, but
> > > > > > it's fast, complete, and has the ability to handle things like
> > > > > > gap buffers.
> > > > >
> > > > > Last time I looked, many years ago, there were several bugs (or
> > > > > differences between TRE and POSIX anyway)
> > > >
> > > > Bugs yes, but the POSIX conformance was pretty strict. As I
> > > > said, he seemed to have abandoned the thing, but the bones are
> > > > pretty good. Someone could just fork it and implement all the
> > > > patches people have been contributing that have been languishing
> > > > in the github issue trackers.
> > >
> > > Oh, and re2 isn't built for POSIX compliance. It has no constructs
> > > that require backtracking etc.
> >
> > No DFA engine can do backtracking.
>
> Correct, but there are regexp libraries that will switch between DFA
> and non-DFA implementations depending on whether the regexp has
> constructs in it that require backtracking

Sure, but the DFA and non-DFA engines can vary independently so long as 
they understand the same language, modulo backreference support.

IMHO, back reference support is an old unix mistake. I can't think of a 
time when I've wanted to actually use them, but their theoretical use is 
still a hindrance. One option would be to just deprecate them.