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      To: jimmey@lexis-nexis.com (Jimmey Todd)
      Subject: Re: Flex performance question
      In-reply-to: Your message of Thu, 19 Feb 1998 11:01:17 PST.
      Date: Thu, 19 Feb 1998 15:42:25 PST
      From: Vern Paxson <vern>
 
      So there are several problems.
 
      First, to go fast, you want to match as much text as possible, which
      your scanners don't in the case that what they're scanning is *not*
      a <RN> tag.  So you want a rule like:
 
      	[^<]+
 
      Second, C++ scanners are particularly slow if they're interactive,
      which they are by default.  Using -B speeds it up by a factor of 3-4
      on my workstation.
 
      Third, C++ scanners that use the istream interface are slow, because
      of how poorly implemented istream's are.  I built two versions of
      the following scanner:
 
      	%%
      	.*\n
      	.*
      	%%
 
      and the C version inhales a 2.5MB file on my workstation in 0.8 seconds.
      The C++ istream version, using -B, takes 3.8 seconds.
 
      		Vern

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