From: Ralph Corderoy <email@example.com>
Subject: [COFF] Requesting thoughts on extended regular expressions in grep.
Date: Tue, 07 Mar 2023 17:34:51 +0000 [thread overview]
Message-ID: <20230307173451.D94B421C9B@orac.inputplus.co.uk> (raw)
> I'm sure that with enough effort, it _is_ possible to make a 50K RE
> that is understandable to mere mortals. But it begs the question: why
It could be the quickest way to express the intent.
> The answer to that question, in my mind, shows the difference between
> a clever programmer and a pragmatic engineer.
I think those two can overlap. :-)
> I submit that it's time to reach for another tool well before you get
> to an RE that big
Why, if the grammar is type three in Chomsky's hierarchy, i.e. a regular
grammar? I think sticking with code aimed at regular grammars, or more
likely regexps, will do better than, say, a parser generator for a
type-two context-free grammar.
As well as the lex(1) family, there's Ragel as another example.
> 3. Optimizing regular expressions. You're still bound by the known
> theoretical properties of finite automata here.
Well, we're back to the RE v. regexp again. /^[0-9]+\.jpeg$/ is matched
by some engines by first checking the last five bytes are ‘.jpeg’.
$ debugperl -Dr -e \
> '"123546789012354678901235467890123546789012.jpg" =~ /^[0-9]+\.jpeg$/'
Matching REx "^[0-9]+\.jpeg$" against "123546789012354678901235467890123546789012.jpg"
Intuit: trying to determine minimum start position...
doing 'check' fbm scan, [1..46] gave -1
Did not find floating substr ".jpeg"$...
Match rejected by optimizer
Freeing REx: "^[0-9]+\.jpeg$"
Boyer-Moore string searching can be used. Common-subregexp-elimination
can spot repetitive fragment of regexp and factor them into a single set
of states along with pairing the route into them with the appropriate
The more regexp engines are optimised, the more benefit to the
programmer from sticking to a regexp rather than, say, ad hoc parsing.
The theory of REs is interesting and important, but regexps deviate from
it ever more.
next prev parent reply other threads:[~2023-03-07 17:34 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-02 18:54 Grant Taylor via COFF
2023-03-02 19:23 ` [COFF] " Clem Cole
2023-03-02 19:38 ` Grant Taylor via COFF
2023-03-02 23:01 ` Stuff Received
2023-03-02 23:46 ` Steffen Nurpmeso
2023-03-03 1:08 ` Grant Taylor via COFF
2023-03-03 2:10 ` Dave Horsfall
2023-03-03 3:34 ` Grant Taylor via COFF
2023-03-02 21:53 ` Dan Cross
2023-03-03 1:05 ` Grant Taylor via COFF
2023-03-03 3:04 ` Dan Cross
2023-03-03 3:53 ` Grant Taylor via COFF
2023-03-03 13:47 ` Dan Cross
2023-03-03 19:26 ` Grant Taylor via COFF
2023-03-03 10:59 ` Ralph Corderoy
2023-03-03 13:11 ` Dan Cross
2023-03-03 13:42 ` Ralph Corderoy
2023-03-03 19:19 ` Grant Taylor via COFF
2023-03-04 10:15 ` [COFF] Reading PDFs on a mobile. (Was: Requesting thoughts on extended regular expressions in grep.) Ralph Corderoy
2023-03-07 21:49 ` [COFF] " Tomasz Rola
2023-03-07 22:46 ` Tomasz Rola
2023-03-03 16:12 ` [COFF] Re: Requesting thoughts on extended regular expressions in grep Dave Horsfall
2023-03-03 17:13 ` Dan Cross
2023-03-03 17:38 ` Ralph Corderoy
2023-03-03 19:09 ` Dan Cross
2023-03-03 19:36 ` Grant Taylor via COFF
2023-03-04 10:26 ` Ralph Corderoy
2023-03-03 19:06 ` Grant Taylor via COFF
2023-03-03 19:31 ` Dan Cross
2023-03-04 10:07 ` Ralph Corderoy
2023-03-06 10:01 ` Ed Bradford
2023-03-06 21:01 ` Dan Cross
2023-03-06 21:49 ` Steffen Nurpmeso
2023-03-07 1:43 ` Larry McVoy
2023-03-07 4:01 ` Ed Bradford
2023-03-07 11:39 ` [COFF] " Ralph Corderoy
2023-03-07 18:31 ` [COFF] " Grant Taylor via COFF
2023-03-08 11:22 ` Ed Bradford
2023-03-07 16:14 ` Dan Cross
2023-03-07 17:34 ` Ralph Corderoy [this message]
2023-03-07 18:33 ` Dan Cross
2023-03-07 4:19 ` Ed Bradford
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