Add header for regular expression support.
Add checks for POSIX regex support.
Revert "Add checks for POSIX regex support."

This reverts commit d96a0f80209105e5cbeb6dcf4cf61d9a937a06c8.

The set of people who have support for POSIX regexen but don't want PCREs is
too small to warrant the extra development overhead of supporting multiple
regex engines (like TglNG does). We'll just stick to PCRE8 since that's the
most universal.
Initial implementation of regex support.
Add regex support to patterns.