Regexes behave differently on different OSes

[frasertweedale]

Describe the bug
Regex matching can behave differently on different OSes. This basically comes down to
how the system regex(3) library behaves. e.g. on GNU+Linux (glibc), backslash character
classes (e.g. \s for whitespace, \w for alnum + underscore) work fine, but they are not
recognised on FreeBSD and you are restricted to POSIX definitions (e.g. [[:alpha:]]).

Expected behavior
We need to decide on this - whether we want to normalise the behaviour across OSes by switching regex library to something that behaves consistently, or whether we leave it as a caveat and simply document it clearly.

[frasertweedale]

Urgh, hitting this hard now, trying to test purebred on FreeBSD.