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From: allbery@NCoast.ORG (Brandon S. Allbery KB8JRR/AA)
Newsgroups: comp.lang.perl
Subject: Re: pattern matching performance
Message-ID: <1991May6.225810.20985@NCoast.ORG>
Date: 6 May 91 22:58:10 GMT
References: <1991May6.161906.21161@ccu.umanitoba.ca>
Reply-To: allbery@ncoast.ORG (Brandon S. Allbery KB8JRR/AA)
Followup-To: comp.lang.perl
Organization: North Coast Public Access Un*x (ncoast)
Lines: 27

As quoted from <1991May6.161906.21161@ccu.umanitoba.ca> by rahardj@ccu.umanitoba.ca (Budi Rahardjo):
+---------------
| a pattern in perl. I have a big flatfile (around 17000 lines).
| Using UNIX grep takes around 1 or 2 second, but perl's pattern
| matching takes 12 secs. 
| 
| while (<FLAT>) {
|     if (/$pat/) { print;}
|     }
+---------------

The problem is that /$pat/ gets recompiled for every line.  If you never
change the value of $pat, the sequence /$pat/o will be compiled only once.
If you plan to change it occasionally, you need to be a bit sneakier:

eval 'sub match { print if /$pat/o; }';

Run this every time you change the value of $pat, then call &match to do
the comparison.  (Since the line is already in $_, just say "&match;" to make
it even faster.)  (I think.)

++Brandon
-- 
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