The different meanings of LaTeX's \stop command

Question

we know that

\stop

will cause LaTeX to stop parsing and output what it has processed up to this command.

But then I discovered another way to use it, as demonstrated in this minimum working example:

\documentclass{article}
  \def\demo#1-#2\stop{#2}
\begin{document}
  \def\sample{foo-bar}
  \expandafter\demo\sample\stop
  There's the Word 'bar' above this text and
  this made it into the output despite a stopping command.
\end{document}

How is \stop used here and why didn't it actually interrupt the LaTeX?

Related: No \makeatletter required? In that post, the @ is also used as a delimiter token. — Werner, Nov 06 '12 at 16:42

score 12 · Accepted Answer · answered Nov 06 '12 at 16:34

The meaning of \stop is never actually read in the above. It is just used as a delimiter token. In this situation, absolutely anything will do:

\documentclass{article}
  \def\demo#1-#2\StopInTheNameOfTeX{#2}
\begin{document}
  \def\sample{foo-bar}
  \expandafter\demo\sample\StopInTheNameOfTeX
  There's the Word 'bar' above this text and
  this made it into the output despite a stopping command.
\end{document}

A simple \show\StopInTheNameOfTeX shows that this has no meaning, but in this context that meaningless is itself meaningless.

D'Oh, so much for good choice of token names ... and kudos for \StopInTheNameOfTeX — arney, Nov 06 '12 at 16:47

score 10 · Answer 2 · answered Nov 06 '12 at 16:50

You are defining a macro with delimited argument, something that the primitive commands \def, \gdef, \edef and \xdef can do, but \newcommand can't.

Let's see:

\def\demo#1-#2\stop{#2}

The parameter text consists of the following tokens

#1 • - • #2 • \stop

Here I use • just to separate the tokens and the spaces are for readability; in the context of the parameter text it's common to consider #1 as just one token.

This means that the first argument to \demo will be all that comes after \demo up to (and not including) the first - token at the same brace level; the second argument will be all after that - up to (and not including) \stop, again at the same brace level. So, if the call is

\demo 1{2-3}4-5\stop

TeX will set #1 <- 1{2-3}4 and #2 <- 5

The meaning of the delimiter tokens is never looked at when \demo is expanded: TeX just matches the pattern, taking the shortest match. For instance, with

\demo--\stop

we'll have an empty #1 and #2 will be changed into -.

You can even use an undefined token as delimiter:

\def\demo#1-#2\nobodydarestodefinethistoken{#2}

and TeX won't complain: the pattern is matched, parameters are substituted with the found arguments and the tokens used for the match will be discarded.

There is one exception: the left brace that starts the replacement text can be used also as a delimiter with a peculiar syntax: if you say

\def\demo#1-#2#{#2}

and then

\demo ABC-DEF{}

the second argument will be DEF and the left brace that must follow will not be discarded.

The different meanings of LaTeX's \stop command

2 Answers2