Re: [9fans] non greedy regular expressions

["Brian L. Stuart" <blstuart@bellsouth.net>]

> The set of "big books on regular expressions" includes Jeffrey Friedl's
> "Mastering Regular Expressions" that happens to contain a chapter by the
> title "NFA, DFA, and POSIX" wherein he says:
>
> > DFA Speed with NFA Capabilities: Regex Nirvana?

This guy seems to blur the distinctions here.  His discussion
makes it sound like he's saying that NFAs have more expressive
power than DFAs.  This is incorrect.  Both NFAs and DFAs have
exactly the same expressive power as the class of grammars
called regular.  For the arbitrary case of nesting (e.g. parens),
these machines are insufficient.  However, for any prescribed
maximum nesting level, you can write a regular expression to
account for it, though it becomes clumsy.

To get more expressiveness, you need to go to a machine
with more functionality.  Classically, the next step
up the hierarchy is the pushdown automaton.  The expressive
power of this machine corresponds directly to the context-
free grammars.  Because the full generality of the CFG
require nondeterminism, automated translations from CFG
to code/machine are usually done with restricted classes
of CFGs, such as LR(k) and LALR.  You can also increase
the power of a FA by adding a counter or by making the
transitions probablistic.  If you truly want to build
expression matching mechanisms that go beyond regular,
building on the FA with counter(s) would be a far more
sound foundation than a lot of the ad hoc stuff that's
been done.  But the truth is that whipping up a CFG
and feeding it to yacc is far more expedient than torturing
regular expressions all day.

> Again, turns out the "big books on regular expressions" can give the
> lowlife--that's me--things "hackers" deny them.

And a good book on automata theory can give you far more
than any "big book of...", "... in 21 days" or "... for
dummies" book can.  Besides, why would you think that
anyone is denying you knowledge or the opportunity
to write code that demonstrates your ideas?

BLS