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.