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

[Paul Eggert]

Vincent Lefevre wrote:
> I wonder whether anyone is interested in matching individual bytes
> in a file regarded as UTF-8 encoded. This seems weird.

It's not weird at all.  For example, suppose we invent the notation 
[[:error:]] to match encoding errors.  Then the pattern '[[:error:]]' 
would match all encoding errors in a file, which could well be a useful 
thing.

Currently, for example, the tz package <http://www.iana.org/time-zones> 
has a Make rule 'check_character_set' that verifies that the source 
files are all properly encoded.  It executes this shell command:

! grep -nv '^.*$' file names

This relies on GNU grep's behavior that "." does not match an encoding 
error.  But it's a command that is not obvious.  It'd be simpler and 
clearer to write this:

! grep -n '[[:error:]]' file names

if such a feature were available.