In the following pdftex example we make verbatim environment inside "...", i.e., all special characters get catcode 12. The problem is with {:


    \endlinechar=-1 \scantokens{#1}%

hello "{" world

Why it gives this error?

Runaway argument?
{" world 
! Forbidden control sequence found while scanning use of ".
<inserted text> 
<to be read again> 
l.13 \bye


Should not it tokenize arguments until it encounters the second "?

How to make \def"#1"{...} work in all cases?

  • Well braces can still group arguments, e.g. \def"#1#2"{arg1: #1 arg2: #2} and "{abc}d" will give arg1: abc arg2: d. So braces must be balanced. Check the definition of \verb in latex.ltx for a solution. – Ulrike Fischer Aug 10 '16 at 7:48

When TeX finds ", it looks for the next " at the same group level, which it doesn't find, because { still has its usual category code 1. Since " is not \long, the first \par token triggers the error.

You have to split processing in two parts: first set the category codes, then absorb the argument.

The parameter text for a macro can define both delimited or undelimited arguments, but, in any case, TeX keeps track of <left brace> and <right brace> (meaning explicit character tokens of category code 1 and 2 respectively).

So, for instance,

\def\foo (#1,#2){#1-#2}

defines a macro that must be followed by (12, with the first argument delimited by ,12 and the second argument delimited by )12.

However, the call


would absorb #1 as {(a,b)} (and the braces would be removed), and #2 as c.

In your case, only balanced braces would take you home.


I'd not use \scantokens and not even make " scan for an argument at all but set the appropriate catcodes in a group and let " to \endgroup (also works with Knuth TeX).



hello "{}%~&^_$#\" world

If you want to have it in a box, use some two-stage method.



hello "{}%~&^_$#\" world

Knuth uses something similar in manmac.tex.

  \obeyspaces \obeylines \tt}

{\obeylines \gdef|{\ttverbatim \spaceskip\ttglue \let^^M=\  \let|=\endgroup}}
  • 2
    Which is a simplified version of what \verb does in LaTeX. – egreg Aug 10 '16 at 8:46
  • The reason that I use \scantokens is to be able to use active quotes in arguments to a macro. See this question tex.stackexchange.com/questions/317879/how-is-catcode-handled Your solution will not work in such case. – Igor Liferenko Aug 10 '16 at 8:49
  • @IgorLiferenko Okay, but then you can't use it to typeset verbatim content in the argument of a macro. I think that's just not possible. The unmatched brace is kind of harmless, but try forwarding %. – Henri Menke Aug 10 '16 at 8:55
  • @IgorLiferenko You could typeset the verbatim material at global scope and save it in a box. Then you can pass the box to a macro without having to worry. – Henri Menke Aug 10 '16 at 8:56

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