SvPV() does not take a const SV*, which means that the pattern argument

SvPV() does not take a const SV*, which means that the pattern argument
to Perl_re_compile() can't be const, which means that the pattern
argument to Perl_pregcomp() can't be const, as can't the argument in
the function in the regexp engine structure.

It's a shame that no-one spotted this earlier.
(Again) I may have rendered the documentation inaccurate.

p4raw-id: //depot/perl@34672