(flex.info)unnamed-faq-75

---

Next: unnamed-faq-76 Prev: unnamed-faq-74 Up: FAQ

---

Enter node , (file) or (file)node

unnamed-faq-75
==============

     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