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Re: [Gnu-arch-users] Preventing matches in regular expressions
From: |
Tom Lord |
Subject: |
Re: [Gnu-arch-users] Preventing matches in regular expressions |
Date: |
Wed, 11 Aug 2004 01:13:30 -0700 (PDT) |
> From: Jan Hudec <address@hidden>
> Lookahead/lookback assertions are perfectly regular.
For the most part, yes.
> They definitely can be expressed as a NFA. And negative
> lookahead assertion is all you need.
Hence as a dfa, yes.
There's the rub.
In an NFA, you can express boolean regexp logic in a very compact
amount of space, but at the cost of backtracking (or parallel
execution). In a DFA (or quasi-DFA, like Rx), you get boolean logic
at the cost of a lot of space.
Historically, the tagging method regexps use a very limited form of
boolean logic. Fortunately, they use a kind that can be optimized by
`cut'. The reason they use boolean logic historically is because
`inventory' was originally a call to `find(1)' (and I was assuming GNU
find); that was the easy way to write it. The reason `inventory'
happens to be optimizable using cut is, alas, shear luck: I wasn't
thinking about that at the time; in fact, I was underestimating the
importance of the performance of `inventory' for overall arch
performance.
What I've learned is that `inventory' performance is crtiical. I have
seriously considered, from time to time, arranging to have
=tagging-method piped through `lex(1)' or something similar! Thus,
fully general boolean regexp logic is not likely to be supported, at
least until you and I are close to or past retirement.
-t
- Re: [Gnu-arch-users] Preventing matches in regular expressions, (continued)
Re: [Gnu-arch-users] Preventing matches in regular expressions, Tom Lord, 2004/08/11
Re: [Gnu-arch-users] Preventing matches in regular expressions, Andrew Suffield, 2004/08/10
Re: [Gnu-arch-users] Preventing matches in regular expressions, Jan Hudec, 2004/08/11
Re: [Gnu-arch-users] Preventing matches in regular expressions, Tom Lord, 2004/08/11
Re: [Gnu-arch-users] Preventing matches in regular expressions, Tom Lord, 2004/08/11