# Can I have a delimited macro with a literal # in the parameter text?

I'm trying to define a macro which grabs everything until the next # (parameter token).
My twisted imagination wants something like this:

\def\test#1###2{(#1)[#2]}
\test hello#{world}

to grab hello in #1 (delimited by #) and world in #2 (brace delimited) and then print

(hello)[world]

However I'm failing (miserably) because no matter what combination of ## I try, TeX yells back:

! Parameters must be numbered consecutively.
##
l.1 \def\test#1##
#2{(#1)[#2]}
?

so I guess that simply writing down the # in there is not the way to go.

Is it possible somehow to have a #-delimited macro?

• why the # not simply \test hello{world} ? :-) – David Carlisle May 17 at 11:33
• @DavidCarlisle I was trying to scan the parameter text of a macro looking for its arguments one by one. – Phelype Oleinik May 17 at 11:38
• it's hard (not really possible) to even find out how many arguments a macro has, tex.stackexchange.com/questions/305806/… – David Carlisle May 17 at 11:45
• "Scan parameter text of a macro" -- if this means evaluating the result of \meaning\macro: With \meaning you don't have information about category codes. The meaning of the following macros looks the same but the 1st one does process two args and the 2nd one has just a delimiter and does not process args: 1) \def\macro#1#2{#1 text #2} 2) \catcode`\#=12\relax\def\macro#1#2{#1 text #2} . Also, you are not bound to using hashes for denoting args. You can use any character after assigning catcode 6 to it. You can also use control-sequences/active chars let equal to catcode-6-chars. – Ulrich Diez May 17 at 20:26
• @UlrichDiez Hm, now I see I did not phrase my question properly. What I wanted to achieve (and already changed my mind) was to scan a definition before the actual definition took place (something like \scandef\def\test#1{something with #1}), not with \meaning, so the hashes do have catcode 6 and, in this case, it doesn't matter which character they are because TeX will not allow this. Thanks for the input, though :-) – Phelype Oleinik May 17 at 20:35

The TeXbook, page 203, says in the first doubly dangerous paragraph

Now that we have seen a number of examples, let’s look at the precise rules that govern TeX macros. Definitions have the general form

\def⟨control sequence⟩⟨parameter text⟩{⟨replacement text⟩}

where the ⟨parameter text⟩ contains no braces, and where all occurrences of { and } in the ⟨replacement text⟩ are properly nested. Furthermore the # symbol has a special significance: In the ⟨parameter text⟩, the first appearance of # must be followed by 1, the next by 2, and so on; up to nine #’s are allowed.

There is no way for the parameter text to contain a (category code 6) #, because of the rule stated above.

As usual in the TeXbook, this is not the complete truth; in the second doubly dangerous bend on page 204 one reads

A special extension is allowed to these rules: If the very last character of the ⟨parameter text⟩ is #, so that this # is immediately followed by {, TeX will behave as if the { had been inserted at the right end of both the parameter text and the replacement text. For example, if you say ‘\def\a#1#{\hbox to #1}’, the subsequent text ‘\a3pt{x}’ will expand to ‘\hbox to 3pt{x}’, because the argument of \a is delimited by a left brace.

However, this special extension has no favorable consequence towards your aim.

• Oh, must be followed :/ – Phelype Oleinik May 17 at 12:01
• @PhelypeOleinik Yes, not the weaker “should” or “ought to”. – egreg May 17 at 12:07
• although (not unusually for the texbook) that rule isn't actually completely true (you can follow the last # by { rather than a digit). So that dangerous bend on its own wouldn't be enough to confirm it wasn't possible (but it isn't possible:-) – David Carlisle May 17 at 12:11
• @DavidCarlisle Yes, I added the relevant quotation. – egreg May 17 at 12:15
• There is module 476 in tex.pdf, which I sort of understand (because I already know what it does (sort of)). It first looks for a left brace, if it's not then steps a counter t and if the grabbed token isn't equal to t the error message in the question is printed. – Phelype Oleinik May 17 at 12:20

You can't really do what you ask, but you can ignore the # while parsing the arguments, then get rid of it:

\def\test#1#{\def\tmp##1{#11}\zz}
\def\zz#1{(\tmp{})[#1]}

\test hello#{world}

\bye
• “You can't really do” is acceptable (although frustrating). Is there anywhere that explains this (not that I'm doubting you ;-)? – Phelype Oleinik May 17 at 11:58
• @PhelypeOleinik egreg's answer shows some sort of documentation, although as i commented there the texbook often"clarifies" rules later so it is hard to use it as a definitive source, there is always tex-the-program..... – David Carlisle May 17 at 12:13