> It is close to impossible to check that a change like this doesn't break
> existing scripts.  And when something breaks, e.g. a syntax file, a
> normal user is very unlikely to be able to figure out what caused the
> problem.

So what about introducing a flag `\V` (or similar) at the start of an
regex that triggers the new engine? As the faster regex behaviour is
mainly needed in syntax files, there could be `syntax fastmatch` etc
commands that have the semantics of the new engine (and which
guarantee deterministic run times if I understand correclty) and give
hard errors on unsupported stuff such as backreferences.

To show that this would make a big difference, solid benchmark data
would be valuable. Has anyone benchmarked the new engine?

If the new engine is supposed to have Posix-compatible behaviour, it
should be able to pass these tests: http://www.research.att.com/~gsf/testregex/
(you'd need to convert the test regexen from posix syntax to vim
syntax first).

Nico
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