pub fn is_meta_character(c: char) -> bool
Returns true if the give character has significance in a regex.
These are the only characters that are allowed to be escaped, with one
exception: an ASCII space character may be escaped when extended mode (with
x flag) is enabld. In particular,
is_meta_character(' ') returns
Note that the set of characters for which this function returns
false is fixed and won't change in a semver compatible release.