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Date: 2005-01-11 (20:30)
From: Gerd Stolpmann <info@g...>
Subject: Re: [Caml-list] Thread safe Str
On Die, 2005-01-11 at 18:55, skaller wrote:
> On Tue, 2005-01-11 at 23:08, Gerd Stolpmann wrote:
> > Capturing just for the purpose of string extraction is not problematic.
> Unfortunately you're wrong. I had in fact hoped this was not
> the case, but having read a couple of paper on it,
> I now know that, unfortunately, they are. (See below).

It's a matter of taste. Granted, you cannot define capturing directly
for an automaton, only by a possibly poor backtracking algorithm. (This
means the approach is "definition by algorithm", not really the best way
to do it.)

Which leads to a more serious problem: One must know the algorithm of
the implementation to avoid running into a hard backtracking case (which
can be exponential). There are a lot of tutorials explaining good and
bad regexps. The lex-type scanners do not have this problem (but you may
run into space problems instead because the automaton becomes large).

> Alain Frisch is one of the leaders in this area, perhaps
> he can explain it better. You can get Cardelli and Frisch
> paper here:
> http://felix.sf.net/papers/greedy.pdf

When I understand the direction correctly, this paper explains how to
avoid backtracking by using a two-pass algorithm (automaton plus
expression-based postprocessing to get the captured strings).

> another by Ville Laurikari here:
> http://felix.sf.net/papers/spire2000-tnfa.ps
> and his Master thesis on the topic here:
> http://felix.sf.net/papers/regex-submatch.ps
> [and if you can figure out how to actually build
> a tagged DFA after reading any of that please let
> me know .. ]
> Frisch's algorithm used in CDuce works with a forward
> pass that ignores captures, but records the automaton states,
> and then a backwards pass the extracts the information
> (CDuce actually builds trees).
> Unfortunately this has a problem: it requires a
> bidirectional iterator, whereas a DFA only requires
> an input iterator. (NFA's require forward iterators)

For string regexps this is not a big problem, the string is in memory.
For XML, this is more a problem, especially when the XML document is
only available event by event (processing large documents).

Gerd Stolpmann * Viktoriastr. 45 * 64293 Darmstadt * Germany 
gerd@gerd-stolpmann.de          http://www.gerd-stolpmann.de