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

Philip Taylor (Hellenic Institute) P.Taylor at Hellenic-Institute.Uk
Sun Apr 16 18:44:19 CEST 2023

On 16/04/2023 17:37, Jonathan Kew wrote:
> First note: I see the same result with plain TeX. So not a XeTeX issue.
Agreed, but as I use only XeTeX this seemed the right place to ask. I 
did test with PdfTeX (not having plain TeX in my TeXworks armoury) and 
PdfTeX did much the same but put a real <tab> in the transcript as 
opposed to ^^I.
> Anyway, this is expected behavior. The ^^I isn't part of the offending 
> control sequence; it's just the preceding context on the line, which 
> is what normally appears in a TeX error message.
OK, so I should have realised that in the absence of a leading "\", ^^I~ 
could not possibly be a control sequence and therefore the "~" had to be 
an active character.  That fact had passed me by ...
> Ignored characters are not "removed from the input" (despite anything 
> Eijkhout says); they're still present, just ignored.

Having consulted Eijkhout first, I did then search the TeXbook to see if 
I could find a definitive statement concerning the treatment of ignored 
characters but failed to do so — perhaps I should search the PDF version 
rather than the printed ...

/Philip Taylor/

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