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Date: 2005-01-11 (19:58)
From: Chris King <colanderman@g...>
Subject: Re: [Caml-list] Thread safe Str
Forgot to CC this to the list, sorry if anyone gets a dupe....

> I'm sure you'd agree there are separable issues here:
> (1) using a string encoding of a regexp as opposed to
> a lex like one -- this has nothing to do with
> captures.

Yes.  As I stated in another e-mail in this thread, I'd love to see an
API that exposes the parse tree of a regexp.

> (2) Captures

I'd like to add (3) Parsing vs. substitution.  You can't effectively
do the latter without captures (of course it can be done but it's

> The fact that the regexp syntax is not checked statically
> isn't relevant in a dynamic language since the typing
> of the rest of the program isn't either.

I think we're talking about different things... I used the "s//g"
syntax to represent the substitution function in whatever language is
being used, not as an example of something to be compiled.  (regexp
"foo(.*)bar") certainly has a static type, and if it's malformed it
simply raises an exception.

> I'm trying to provide that in Felix. It has Python style literals,
> and Python style substrings. However it is still clumbsy compared
> with Perl (I guess .. I can't write Perl ..)

Perl string mangling is clumsy compared with Python.  The key is that
Python treats strings as arrays (or lists) of characters.

> > True.  Performing multiple replacements on a single string with a
> > regexp is retarded.  But so is writing a lexer for a simple one-shot
> > replacement job.
> That depends on how hard it is to write a lexer.

Specifically, a lexer whose input and output are both strings and
which performs substitution.  Not pretty, unless captures are

> At present, Felix regexps have to be constants. It will be possible
> in the bootstrapped compiler to generate them as text, and then
> compile and link (i.e. there will be a function sort of like 'eval').

That's not acceptable for, say, an incremental search, though, where a
new regexp must be generated on each keystroke. (Yes I know regexps
aren't the best way to go about that but I'm sure there are better

> >  My point is just that regexps are useful enough to co-exist with lexers.
> But they're the same thing. Lexer provide regular definitions,
> which is just a way of naming regexps, and reusing the regexps
> by providing the name.

Not in the form of lex/flex/ocamllex.  Yes, lexer token definitions
are equivalent to regexps, but everything else about them is different
(specifically, lexers are event-based, and don't provied captures).
I'd love to see a regexp engine allowing dynamic creation of
token-based regexps, complete with captures.  It could easily serve as
both the base of a lexer and a substitution engine.  Heck, that sounds
like a fun project... what I am doing this weekend? :P