Tweak regex to avoid a bug in Perl 5.16.3.

Tweak regex to avoid a bug in Perl 5.16.3.

For some reason, 5.16.3 (and perhaps slightly earlier/later versions)
go into an infinite loop with the version-replacement regex installed
by commit fc0d0ce97.  We can work around that by using an explicit
"\n" instead of the line-start metacharacter "^".

Reported-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0u9dV3CdKqkqdusA_RdvBkwWe0c0rxcFWj++VYoutFYSw@mail.gmail.com