ocamllex -- ungetting a character?

ocamllex -- ungetting a character?

[mvanier]

I'm writing a simple program using ocamllex to do some text processing, and one 
feature I would really like is to be able to put a matched character back onto 
the front of the lexing buffer.  I can't figure out a way to do this from the 
documentation (if it's possible, it's not in the manual).  Does anyone know of a 
way to do this?

Thanks in advance,

Mike

Re: [Caml-list] ocamllex -- ungetting a character?

[Stefano Zacchiroli]

On Sat, Jul 15, 2006 at 08:57:51PM -0700, mvanier wrote:
> I'm writing a simple program using ocamllex to do some text processing, and 
> one feature I would really like is to be able to put a matched character 
> back onto the front of the lexing buffer.  I can't figure out a way to do 
> this from the documentation (if it's possible, it's not in the manual).  
> Does anyone know of a way to do this?

I don't think it is possible with ocamllex. Using ulex however you can
put back all the characters corresponding to the last matched lexeme
with Ulexing.rollback.

Cheers.

-- 
Stefano Zacchiroli -*- Computer Science PhD student @ Uny Bologna, Italy
zack@{cs.unibo.it,debian.org,bononia.it} -%- http://www.bononia.it/zack/
If there's any real truth it's that the entire multidimensional infinity
of the Universe is almost certainly being run by a bunch of maniacs. -!-

Re: [Caml-list] ocamllex -- ungetting a character?

[Jean-Christophe Filliatre]

Stefano Zacchiroli writes:
 > On Sat, Jul 15, 2006 at 08:57:51PM -0700, mvanier wrote:
 > > I'm writing a simple program using ocamllex to do some text processing, and 
 > > one feature I would really like is to be able to put a matched character 
 > > back onto the front of the lexing buffer.  I can't figure out a way to do 
 > > this from the documentation (if it's possible, it's not in the manual).  
 > > Does anyone know of a way to do this?
 > 
 > I don't think it is possible with ocamllex. 

Well, there  is at least an  ugly way to  do that, which is  to modify
lexbuf.lex_curr_pos manually, as in

======================================================================
  | <your regexp>   { lexbuf.lex_curr_pos <- lexbuf.lex_curr_pos - 1;
	              <your action> }
======================================================================

I've already used this with success  (yes, I feel ashamed :-). I would
like a  member of  the ocaml  development team to  confirm it,  but it
seems  safe, as long  as you  do not  backtrack before  the characters
matched by the  left hand-side regexp. Indeed, the  lexing engine will
not modify the lexbuf once the action is triggered and before the next
call to the lexer.

Hope this helps,
-- 
Jean-Christophe Filliâtre (http://www.lri.fr/~filliatr)