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Re: Encoding detection again ...

  • From: David Brownell <db@E...>
  • To: Miles Sabin <msabin@c...>
  • Date: Tue, 02 Mar 1999 13:19:59 -0800

utf 16 ucs 2 detection
Miles Sabin wrote:
> 
> Appendix F of the spec say that given a document
> starting with the 4 octet sequence,
> 
>   00 3C 00 3F
> 
> I'm to infer BOM-less big-endian UTF-16, and
> given a document starting with,
> 
>   3C 00 3F 00
> 
> I'm to infer BOM-less little-endian UTF-16.

That is, the appendix _suggests_ (in a non-normative
fashion) that's the way to go.


> What I what to know is: why could these
> sequences not equally represent (respectively)
> big-endian UCS-2 or little-endian UCS-2?

They could ...

> 
> 1. Unicode == UTF-16
> 2. UCS-2 != UTF-16 (because UCS-2 lacks UTF-16's
>    support for characters outside the BMP).

Put it this way:  if you assume UTF-16, you're
safe either way because UTF-16 is a superset.

It'd be reasonable for an autodetecting algorithm
to support "downgrading" its guess from UTF-16 to
UCS-2, and should probably do so if it's reporting
encoding mismatches as fatal errors.

- Dave

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