SMIE is a package that provides a generic navigation and indentation engine. Based on a very simple parser using an operator precedence grammar, it lets major modes extend the sexp-based navigation of Lisp to non-Lisp languages as well as provide a simple to use but reliable auto-indentation.
Operator precedence grammar is a very primitive technology for parsing
compared to some of the more common techniques used in compilers.
It has the following characteristics: its parsing power is very limited,
and it is largely unable to detect syntax errors, but it has the
advantage of being algorithmically efficient and able to parse forward
just as well as backward. In practice that means that SMIE can use it
for indentation based on backward parsing, that it can provide both
backward-sexp functionality, and that it
will naturally work on syntactically incorrect code without any extra
effort. The downside is that it also means that most programming
languages cannot be parsed correctly using SMIE, at least not without
resorting to some special tricks (see Living With a Weak Parser).