Support regular expressions on non-POSIX platforms

In code there exists a POSIX detection mechanism that conditionally enables POSIX regexes. I'm working on creating a portable solution with C++11. This is similar to #2380.

Start by locating the existing POSIX detection mechanism that conditionally enables POSIX regular expressions. Read issue #2380 for related context and identify the C++11 portability constraints for non-POSIX platforms. Done means regular-expression support works portably on the affected platforms, with corresponding coverage established.

Written by the indexing model from the issue text.