From: cowan@mercury.ccil.org (John Cowan)
Subject: [TUHS] Short history of 'grep'
Date: Sat, 30 Jan 2016 21:37:00 -0500 [thread overview]
Message-ID: <20160131023700.GB7917@mercury.ccil.org> (raw)
In-Reply-To: <alpine.BSF.2.11.1601311227470.14724@aneurin.horsfall.org>
Dave Horsfall scripsit:
> I'm still trying to get my around about how a program such as "egrep"
> which handles complex patterns can be faster than one that doesn't... It
> seems to defeat all logic :-)
Actually, it just appears to be that way. Many egrep things like + and ?
are supported in grep too, you just have to enter them as \+ and \?, at
least now that we have Posix regular expressions. What egrep does not
support that grep does is backreferences, and that allows it to use highly
efficient deterministic (i.e. non-backtracking) finite state automata.
Classic grep uses backtracking, which makes it much slower on problematic
expressions like "a*b" where there is no b in the input. On the other
hand, creating a deterministic automaton has higher setup costs.
--
John Cowan http://www.ccil.org/~cowan cowan at ccil.org
Si hoc legere scis, nimium eruditionis habes.
next prev parent reply other threads:[~2016-01-31 2:37 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-30 3:00 Warren Toomey
2016-01-30 19:10 ` Mary Ann Horton
2016-01-30 19:44 ` Dave Horsfall
2016-01-30 20:20 ` Mary Ann Horton
2016-01-30 20:40 ` Dave Horsfall
2016-01-30 21:42 ` Marc Rochkind
2016-01-31 1:41 ` Dave Horsfall
2016-01-31 1:50 ` Larry McVoy
2016-01-31 2:06 ` jason-tuhs
2016-01-31 4:20 ` Random832
2016-01-31 17:11 ` Mary Ann Horton
2016-01-31 17:38 ` John Cowan
2016-02-01 10:48 ` Tony Finch
2016-01-31 2:37 ` John Cowan [this message]
2016-02-01 10:38 ` Tony Finch
2016-02-01 19:26 ` scj
2016-01-31 17:01 Doug McIlroy
2016-03-05 1:48 ` Dave Horsfall
2016-03-05 1:54 ` Larry McVoy
2016-02-01 20:57 Doug McIlroy
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