lexing backwards

Could anyone point me to work and experience on lexing text locally
and backwards ?


Traditionally lexing is done left-to-right with longest-match regexps.
But such a lexing system obviously means that if you're in the middle
of a file and want to know what is the previous lexing token, you end
up (in the general case) having to lex from the beginning of the file.


Assuming one wants to be able to do such backward lexing and that one
wants to get the answer locally (without having to go back to the
beginning of the file), one needs to put some restrictions on the kind
of tokens (use shortest-match regexp, or use a subset of regexps) one
might accept, or one might just want to add some heuristics, or else
use caching, or ...


I can think of a few different things to do, but I'm hoping someone
out there has some experience with it.




                Stefan


PS: This is in the context of basic Emacs support for major modes, so it
        is not specific to a particular grammar although it's OK if it
        does not cover all possible language syntaxes.

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