In the past weeks i've been thinking over the problem on the practical
problems of regex in its matching power. For example, often it can't
be used to match anything of nested nature, even the most simple
nesting. It can't be used to match any simple grammar expressed by
BNF. Some rather very regular and simple languages such as XML, or
even url, email address, are not specified as a regex. (there exist
regex that are pages long that tried to match email address though)
I wrote out a more elaborate account of my thoughts here: http://xahlee.org/cmaci/notation/pat...tern_spec.html
----------------
After days of researching this problem, looking into parsers and its
theories etc, today i found the answer!!
What i was looking for is called Parsing Expression Grammar (PEG).
See http://en.wikipedia.org/wiki/Parsing_expression_grammar
It seems to me it's already in Perl6, and there's also a
implementation in Haskell. Is the perl6 PEG is in a usable state?
Thanks.
Xah xa*@xahlee.org
∑ http://xahlee.org/
☄ 3  2381 
In article <48************ ***********@nn1-read.tele2.net> ,
Lars Rune Nøstdal <la*********@gm ail.comwrote:
Hi,
Finite automata works for "nested things".
Only in the special case when the depth of nesting is bounded ahead of
time. If it's unbounded then there is an unbounded amount of "stack"
information that the automaton needs to remember, therefore a finite
automaton cannot do it.
<pedantry>
That should be "Finite automata WORK ...", since "automata" is plural.
</pedantry> http://en.wikipedia.org/wiki/Automata_theory
--
---------------------------
| BBB b \ Barbara at LivingHistory stop co stop uk
| B B aa rrr b |
| BBB a a r bbb | Quidquid latine dictum sit,
| B B a a r b b | altum viditur.
| BBB aa a r bbb |
-----------------------------
On Fri, 9 May 2008 22:52:30 -0700 (PDT), "xa****@gmail.c om"
<xa****@gmail.c omwrote:
>In the past weeks i've been thinking over the problem on the practical
problems of regex in its matching power. For example, often it can't
be used to match anything of nested nature, even the most simple
nesting. It can't be used to match any simple grammar expressed by
BNF. Some rather very regular and simple languages such as XML, or
even url, email address, are not specified as a regex. (there exist
regex that are pages long that tried to match email address though)
What's your point? The limitations of regular expressions are well
known.
>After days of researching this problem, looking into parsers and its
theories etc, today i found the answer!!
What i was looking for is called Parsing Expression Grammar (PEG).
PEG has its own problems - it's very easy with PEG to create subtly
ambiguous grammars for which quite legal looking input is rejected.
And there are no good tools to analyze a PEG and warn you of subtle
problems.
Chris Clark (YACC++) has posted at length about the merits, problems
and limitations of various parse techniques - including PEG - in
comp.compilers. Before you consider doing anything with PEG I suggest
you look up his posts and read the related threads.
>It seems to me it's already in Perl6, and there's also a
implementati on in Haskell. Is the perl6 PEG is in a usable state?
Thanks.
Xah
xa*@xahlee.org
? http://xahlee.org/
George
--
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