[XeTeX] Confused (why are ignored characters not "removed from the input" as per Eijkhout's TeX by Topic ?)

Philip Taylor (RHBNC) P.Taylor at Rhul.Ac.Uk
Sun Apr 16 18:16:09 CEST 2023

Given —

\catcode 9 = 9

\let ~ = \undefined



why does XeTeX report

This is XeTeX, Version 3.141592653-2.6-0.999993 (TeX Live 2021/W32TeX) (preloaded format=xetex)

restricted \write18 enabled.

entering extended mode


! Undefined control sequence.

l.3 ^^I~


This output makes it appear that the offending control sequence is (or might be) ^^I~, whereas it is in fact simply ~.  Should not the ^^I have been ignored rather than reported, as per line 1 ?  Note line 3 does not really contain ^^I~ but rather <tab>~, but as tabs cannot be reliably included in e-mail I represent them here as ^^I.  The ^^I in the transcript is genuine.

Philip Taylor

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