Under Typesetting hashtags with natural syntax Joseph Wrigh ask me to ask a new question:

In LaTeX, \makeatletter is short for changing the category code of @. But something like


gives error. What should I do?

Here is the non-working example.


More Question

Based on Joseph's answer, the safest way is


Then it comes into my mind why TeX accepts the unsafe way \catcode`@11? And what should \catcode`\=11 be: is it \catcode`\\=11 or \catcode`\==11? Same problem appears between \let\a\b and \let\a=\b. The worst thing is that and = do not have a specific category code, what if one day I need \makegraveletter or \makeequalsignletter?

  • I'm asking what's the use of # with category code 11. If any, it should be 12. The category code of ` and = is normally 12 (and this category code is required in the syntax of assignments and alphabetic constants). – egreg Sep 17 '14 at 10:04
  • @egerg I just wonder where is the limit. I am not going to play with it in any serious document. (But 12 is required for csname such as \parameter#xxiv, is it reasonable?) – Symbol 1 Sep 17 '14 at 11:16
  • No; catcode 12 is not needed if you use \csname a#b\endcsname (unless it's in the replacement text for \def, where you need ##). – egreg Sep 17 '14 at 11:19
  • @egreg Codes combining \csname, \string, \expandafter, and so and so always drive me crazy. I think I should stop here since I can use fullwidth characters in Unicode. – Symbol 1 Sep 17 '14 at 11:27

The simple answer here is that you need to escape # and use \#: some chars are not 'safe' on their own but the escaped versions always are. A classic example is %, which would start a comment normally but doesn't when you use \%. As David says, TeX always allows the use of single-character commands as part of the backtick syntax in contexts such as `\catcode, so it's always safe to use the escaped version even if not needed.

Taking that knowledge and using the example from the linked question to make a few different possibilities we might end up with something like

\newcommand*\makehashletter{\catcode`\#=11 }
\newcommand*\makehashparameter{\catcode`\#=6 }

Notice that I've added a space after the catcodes in both of the cases here: this stops TeX looking for more digits and is best practice when creating such commands. (I could use a \relax here.)

  • 1
    \begingroup\lccode`~=`#\lowercase{\endgroup\let~}\maketag is much easier – egreg Sep 17 '14 at 9:56
  • @egreg Depends what exactly you want to achieve (if \maketag changes should # or not). – Joseph Wright Sep 17 '14 at 17:23

you can use


The backtick notation takes a single character or a single character control token, for this reason.

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