With REG_NEWLINE, POSIX says:
"A <newline> in string shall not be matched by a period outside
 a bracket expression or by any form of a non-matching list"

musl currently matches newlines with negated brackets, even if
REG_NEWLINE is used. Attached patch fixes the issue, although
I'm not sure if it's the best way to do it.

Also see similar glibc bug:
https://sourceware.org/bugzilla/show_bug.cgi?id=3957