[tex-hyphen] hyphenation different in xelatex?

[Philip Taylor]

Reinhard Kotucha wrote:

> I suppose that TeX 3 is 100% downwards compatible with TeX82. Phil, 
> can you confirm?

Knuth described the differences that affect compatibility (I have
removed those that apply only to METAFONT) in a message sent to Barbara
Beeton during October 1989 :

> 12. Exceptions to upward compatibility. The new features of TEX and 
> METAFONT imply that a few things work differently than before. I
> will try to list all such cases here (except when the previous
> behaviour was erroneous due to a bug in TEX or METAFONT). I don’t know
> of any cases where users will actually be affected, because all of
> these exceptions are pretty esoteric.

> • TEX used to convert the character strings ^^0, ^^1, ... , ^^9, ^^a,
> ^^b, ^^c, ^^d, ^^e, ^^f into the respective single characters p, q,
> ... , y, !, ", #, $, %, &. It will no longer do this if the following
> character is one of the characters 0123456789abcdef.
> 
> • TEX used to insert no character at the end of an input line if 
> \endlinechar>127. It will now insert a character unless 
> \endlinechar>255. (As previously, \endlinechar<0 suppresses the 
> end-of-line character. This character is normally 13 = ASCII 
> control–M = carriage return.)
> 
> • Some diagnostic messages from TEX used to have the notation ["80] 
> ... ["FF] when referring to characters 128 ... 255 (for example when 
> displaying the contents of an overfull box involving fonts that 
> include such characters). The notation ^^80 ... ^^ff is now used 
> instead.
> 
> • INITEX used to forget all previous hyphenation patterns each time 
> you specified \patterns. Now all hyphenation pattern specifications
> are cumulative, and you are not permitted to use \patterns after a 
> paragraph has been hyphenated by INITEX.
> 
> • TEX used to act a bit differently when you tried to typeset missing 
> characters of a font. A missing character is now considered to be a 
> word boundary, so you will get slightly more diagnostic output when 
> \tracingcommands>0.
> 
> • TEX and METAFONT will report different statistics at the end of a 
> run because they now have a different number of primitives.
> 
> • INITEX programs must now set \lefthyphenmin=2 and
> \righthyphenmin=3 in order to reproduce their previous behaviour.

(Retrieved from
http://ftp.math.utah.edu/pub/tex/historic/systems/knuth/tex-impl/message.019.txt)

** Phil.