Merge patch series "regex patches"

Rasmus Villemoes <ravi@prevas.dk> says:

This started as a rather simple patch, 1/12, adding the ability to
more conveniently do regex matching in shell.

But with that, it became very easy to see what the slre library can
and especially what it cannot do, and that way I found both outright
bugs and a "wow, doesn't it support that syntax" gotcha. I couldn't
find any tests ('git grep slre -- test/' was empty), so I added a
small test suite and tweaked slre.c.

Link: https://lore.kernel.org/r/20250513084034.654865-1-ravi@prevas.dk