bug#44351: Bug in grep v3.2 onwards in regular expression matching

[Gonzalo Padrino]

Hello,

  While using GNU grep v3.4 in an Ubuntu 20.04 userspace running on top of
Win10 WSL (yeah, i know... but also checked in other envs) i discovered
what seems like an obvious bug (if i'm not mistaken).
  The bug:
-----
me@host:~$  echo 'xxxxy' |grep -E '^x+x+x+x+y$'
xxxxy
me@host:~$  echo 'xxxy' |grep -E '^x+x+x+x+y$'
xxxy
me@host:~$  echo 'xxy' |grep -E '^x+x+x+x+y$'
xxy
me@host:~$  echo 'xy' |grep -E '^x+x+x+x+y$'

----
...the terminal supports ansi color escapes, and what's really weird is
that only the result from the first command is colored in red. First and
fourth commands yield correct results; the second and third do not, as they
should not match it's input.

  I've tested releases from v3.1 to latest v3.5 and found the anomalous
behaviour in version v3.2 through v3.5. A (quick and clunky) git bisect led
me to believe it was introduced about two years ago, possibly in commit
123620af88f55c3e0cc9f0aed7311c72f625bc82 (
https://git.savannah.gnu.org/cgit/grep.git/commit/?id=123620af88f55c3e0cc9f0aed7311c72f625bc82).
If this is true, it would mean either the bug is in gnulib, or maybe grep
needed to do some kind of extra handling on it's side.

Kind regards. Gonzalo Padrino.

P.S.: I had to patch some things in order to successfully compile the code
after checking out some problematic commits (pragmas to avoid warnings
about "pure" and "noreturn" function attributes, a missing configmake
dependency in bootstrap.conf, etc ).