bug#18266: handling bytes not part of the charset, and other garbage

[Paul Eggert]

Vincent Lefevre wrote:

> There's no reason that '.' matches something that doesn't belong to
> the charset in C locale, but doesn't match in a UTF-8 locale.

In the C locale on GNU/Linux, all byte values are members of the 
charset.  That is why it's OK for '.' to accept that byte in the C 
locale but reject it in a UTF-8 locale.

> It's annoying that now in UTF-8, one can no longer match ISO-8859-1 text

This has been true for quite some time in 'grep', at least with the 
standard matchers.  It may not have been true for -P but that relied on 
undefined behavior that could crash grep, and we can't have that.

It would make sense to add a notation to mean "match any character or 
invalid byte", as an extension.  That'd take some work, though.