Support PCRE2

[ellert]

This Pull request:

Makes it possible to use PCRE2 as an alternative to PCRE

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #11395

[Axel-Naumann]

What we (you, the ROOT team, the users) need to discuss is whether we can deprecate TPRegex altogether, given that std::regex is now (since C++17 is required, as a side effect of kicking out old compilers) guaranteed to be available. Or reimplement it using std:: regex.

Thoughts?

[github-actions]

Test Results

    10 files      10 suites   2d 4h 27m 22s ⏱️
 2 636 tests  2 635 ✅ 0 💤 1 ❌
25 131 runs  25 130 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit f89fc64.

♻️ This comment has been updated with latest results.

[ellert]

Some background:
https://fedoraproject.org/wiki/Changes/PcreDeprecation
https://bugzilla.redhat.com/show_bug.cgi?id=2128369

[andresailer]

pcre2 10.40 (as mentioned in #11395) already comes with a pcre2-config.cmake (https://github.com/PCRE2Project/pcre2/blob/pcre2-10.40/cmake/pcre2-config.cmake.in) file. Can that be used instead of adding FindPCRE2.cmake ?

[ellert]

pcre2 10.40 (as mentioned in #11395) already comes with a pcre2-config.cmake (https://github.com/PCRE2Project/pcre2/blob/pcre2-10.40/cmake/pcre2-config.cmake.in) file. Can that be used instead of adding FindPCRE2.cmake ?

The packaged pcre2 in Fedora and RHEL/CentOS/Rocky/Alma does not ship the cmake files. They are only create/installed if pcre2 is built using cmake. The RPM for Fedora and RHEL are built using autotools, and therefore do not contain the cmake files.

[guitargeek]

Not adding this to the 6.32 milestone, since even Fedora rawhide still has the old pcre for now.

[guitargeek]

What we (you, the ROOT team, the users) need to discuss is whether we can deprecate TPRegex altogether, given that std::regex is now (since C++17 is required, as a side effect of kicking out old compilers) guaranteed to be available. Or reimplement it using std:: regex.

Thoughts?

I wouldn't go into this direction yet. We have a couple of problems with std::regexp, like these symbol clashes with other Python libraries:
#15309

Therefore, provided that it's tested, I'm in favor of merging this PR. I'm adding pcre2 to the CI images, so that we can test it:
root-project/root-ci-images#31

[guitargeek]

Amazing! We can see in the CI that it works:

  -- Looking for PCRE
  -- Found PCRE2: /usr/include (found version "10.32") 

[ferdymercury]

Potential regression: #15986