What characters are active by default in TeX?

Question

Since the character ~ has a catcode of 13, it is considered active. I can (ab)use this by executing something like the code \let~\catcode ~`a8 which assigns the character a a catcode of 8 (for subscript), without having to do \catcode`~13 first.

What other characters are considered active by default in TeX? In other words, what characters have a catcode of 13 by default?

~ is the only one usable in latex. In pdftex all characters above 127 are active but if you mess with them you will break the encoding support. — David Carlisle, May 20 '22 at 23:21
Just noting that your question title says "always active", while your question says "active by default". These are not the same thing, since the catcode of ~ can be changed on a whim. — Steven B. Segletes, May 20 '22 at 23:26
Looks like a rather impractical question, but you can just do a loop to print out catcode of every characters. Last time I check, the discretionary hyphen ("A0) is in non-pdflatex engine. — user202729, May 21 '22 at 07:37

plante · Answer 1 · 2022-05-21T10:38:03.607

plain always has character 12 (form feed) and 126 (tilde) as active which is not surprising.

In LaTeX the situation depends on the engine used. With pdflatex characters 128-225 are active for encoding support; in xelatex and lualatex character 173 (U+00AD "soft hyphen") is declared active and given the meaning \- (discretionary hyphen).

In table form:

format / engine	tex	pdftex	xetex	luatex
plain	12 126	12 126	12 126	12 126
LaTeX	/	1-8 11 12 14-31 126 128-255	12 126 173	12 126 173
conTeXt	/	/	/	124 126
AmS-TeX	/	12 64 126	/	/
OpTeX	/	/	/	126 160

Overall only ~ seems to be universal.

The following test file is used:

\ifx\documentclass\undefined \else
    \documentclass{article}
    \begin{document}
\fi
\csname starttext\endcsname
\newcount\curr
\newcount\upper
\ifx\numexpr\undefined
    \upper="100
    \def\n{^^J}
\else
    \ifx\pdftexbanner\undefined
        \upper="10000
        \edef\n{\Uchar"000A}
    \else
        \upper="100
        \def\n{^^J}
    \fi
\fi
\newlinechar=10
\message{\n\n}
\count0=0
\loop
    \ifnum\catcode\curr=\active
        \message{ \the\curr}
    \fi
    \advance\curr 1
\ifnum\curr<\upper \repeat
\message{\n\n}
\csname stoptext\endcsname \end{document}

What characters are active by default in TeX?

1 Answers1