[cmake-developers] Making your regular expression engine more reliable

[Ben Boeckel]

On Thu, May 18, 2017 at 12:44:57 -0700, Alan W. Irwin wrote:
> So your unit tests for regular expressions obviously missed at least
> this issue. I have no idea what those unit tests are (or even if they
> exist), but one possibility for attempting to wring most of the bugs out
> of your regular expression processor is to adapt some other project's
> regexp test suite. See
> <http://stackoverflow.com/questions/15819919/where-can-i-find-unit-tests-for-regular-expressions-in-multiple-languages>
> for a rather large list of such test suites.

This would be a great addition to CMake and would help with a future
replacement to ensure that we are compatible with what is being used
now.

> Another possibility is simply to forget supporting your own regexp
> engine and adopt someone else's very well regarded regexp engine (such
> as libprng).  I vaguely recall that has been suggested before, but
> since that hasn't happened I presume inertia or NIH syndrome won or
> else there was some strong reason why you didn't go that route.

This has been brought up before. The regex engine used in CMake is *old*
and, after my performance fixes a few years ago, is (or, at least, was)
near the top of that list for various reasons.

The biggest problem facing a replacement is the backwards compatibility.
Do we want to just use a standard set of features (C++14?) and break
unknown number of `MATCHES` expressions? This would require a policy and
the old code would still lurk so that we can support the OLD policy.

Or do we translate from what we support now into a standard language and
then use another engine for that? This is probably the better solution
for CMake, but is probably more code than the current engine.

> From my perspective as a strongly interested CMake user (but not a
> CMake developer or regexp guru) that wants a completely reliable
> regular expression engine for CMake, I don't care which of these two
> approaches you use to achieve that goal.  But I hope my starting
> this topic here will facilitate reaching that goal.

I understand the want for a better engine, but just replacing it
outright isn't really an option.

Thanks,

--Ben