Re: yyless and pattern matching

Chris F Clark <>
25 Mar 2000 02:35:13 -0500

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yyless and pattern matching (Michael B. Allen) (2000-03-23)
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From: Chris F Clark <>
Newsgroups: comp.compilers
Date: 25 Mar 2000 02:35:13 -0500
Organization: The World Public Access UNIX, Brookline, MA
References: 00-03-104
Keywords: lex, comment

My suspicion is that this is simply a bug (mis-feature). In the
versions of lex that I've read (which I believe includes flex but
can't say for certain), the ^ operator is handled by a variable that
keeps the previous character read. I bet yyless only pushes the
character back to the input that flex is reading and does not push it
into the variable used for newline checking. Or on a slight
permutation on that the newline checking character may need to be even
one character farther back and thus the yyless may need to push the
character before the character being pushed back (requiring two
characters of lookback). My apologies for the vagueness of this mail,
but I have not analyzed the code that deeply. Still, this might give
someone enough of a hint to fix the problem.

Hope this helps,

Chris Clark Internet :
Compiler Resources, Inc. Web Site :
3 Proctor Street voice : (508) 435-5016
Hopkinton, MA 01748 USA fax : (508) 435-4847 (24 hours)
[I agree. -John]

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