Re: coupling LALR with a scanner?

"Armel" <armelasselin@hotmail.com> wrote in news:11-09-023@comp.compilers:
> Of course in the presence of something like expr / expr the parser knows
> the / is for a division, and if the / is found where expr is expected it
> knows there is a regular expression coming.
> I don't see how I could hack this into a scanner only. maybe some
> specific preceding tokens. I have pass again some time on that question.


The usual way to do this in a flex+bison parser is with a production like:


expr: { beginREGEXP(); } '/' regexp { endREGEXP(); } '/'


where beginREGEXP and endREGEXP are functions in the flex file that set
the start-state into and out of regexp mode. Note that the actions need
to be BEFORE the parser shifts the '/' token, as an LALR(1) parser may look
ahead one token. Which means that a '/' must be a '/' regardless of the
scanner state.


Note that if you allow additional lookahead in the parser (LR(k) or LR(*) or
any kind of backtracking) this will break badly.


Chris Dodd
cdodd@acm.org

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