Re: coding tags and utf-16

[Kenichi Handa <handa@m17n.org>]

Sorry for the late responce.

In article <dse2i6$d2b$1@sea.gmane.org>, Kevin Rodgers <ihs_4664@yahoo.com> writes:

>> I thought we had discussed this already.  The BOM-encodings should
>> have priority since the likelihood of a misdetection is negligible
>> (the character pair does not make sense at the start of a text in
>> latin-1 in any language): the only thing that can reasonably be
>> expected to happen is that a binary file is detected as utf-16.  Not
>> much of an issue, I'd say.

I've just digged out old mails we exchanged on this topic
(about a year ago).  To my understanding, there was no
clear conclusion.  Here are the extracts:
------------------------------------------------------------
I wrote:
> I think BOM is not that safe because there are many charsets
> who have normal letters at 0xFE and 0xFF.

Jason wrote:
> But what are those characters, and are they likely to appear as a pair 
> at the beginning of the file, and nowhere else?

I wrote:
> Sorry, I don't know.

Dave wrote:
>> Exactly what Windows does for what?  Recognizing a utf-16 registry
>> file when opened in the registry editor?

> Auto-detecting utf-16 generally.  Although I don't think it would give
> false positives on iso-8859 text, I don't know if it could with other
> charsets.
> 
> I could believe that Windows doesn't just go by byte-order-mark in
> some locales where there might be a problem.  If so, it could be
> useful to do the same thing.
------------------------------------------------------------

For instance, I've just googled the two character sequence
of 0xFE 0xFF of koi8 and found several occurrences.

> Exactly.  So why haven't these entries been added to 
> auto-coding-regexp-alist?

> ("\\`\xEF\xBB\xBF" . utf-8)

As far as I know, UTF-8 should not start with this sequence
unless the text really starts with ZWNBSP (very unlikely).

> ("\\`\xFE\xFF" . utf-16-be)
> ("\\`\xFF\xFE" . utf-16-le)

Although it's not clear how safe they are, if no one objects,
I'll add them in auto-coding-regexp-alist.

> ("\\`\x00\x00\xFE\xFF" . utf-32-be)
> ("\\`\xFF\xFE\x00\x00" . utf-32-le)

Emacs doesn't support those encoding for the momemnt.

---
Kenichi Handa
handa@m17n.org