Re: pcre performance, was Re: git log filtering

From: Paolo Bonzini <paolo.bonzini@lu.unisi.ch>
Date: Mar 7, 2007, 18:03 UTC
Message-ID: <45EEFE74.1090309@lu.unisi.ch>
In-Reply-To: <Pine.LNX.4.63.0703071807250.22628@wbgn013.biozentrum.uni-wuerzburg.de>

> Result: external grep wins hands-down. GNU regex loses hands-down. pcre 
> seems to be better than glibc's regex engine, and gains ever so slightly 
> when using NO_MMAP.

Indeed GNU regex 0.12 loses, and that's why it was rewritten for (IIRC) glibc 2.3. Older glibc's use code derived from GNU regex 0.12; but the old GNU regex code is dead in general (maybe it survives in Emacs -- but I don't remember), and the glibc regex code can be used by external programs via gnulib.

glibc is slower than PCRE mostly because it is internationalized. So for example it supports things like stra[.ss.]e matching both strasse and straße in a German locale, or [[=a=]] matching aàáäâ and possibly more variations. In theory. In practice I couldn't make it work while writing this message...

External grep wins hands-down because it's a DFA engine. If the regex uses backreferences (or the above esoteric constructs), however, external grep will not be able to give a definite answer using the fast engine, and will fall back to glibc regex.

Paolo