I am trying to make characters active in math mode, but can't seem to get past the most basic of examples. I would like to define a macro which is passed:

  1. the character to make active and

  2. the macro that should be executed once that macro is encountered.

The MNWE below attempts to make =, + and ! active and color code them. The desired output is that those characters get colored in math mode as. Thus, the input

a=+b $c=+4+d+3!$ e=+f


enter image description here

where the characters are not altered outside of math mode (the leading a=+b and trailing e=+f).


  • There is a separate, but related (as it is in the context of active characters) question in the \MyPlus macro to detect a binary addition + versus a unary positive +. Perhaps that should be a separate question?

Related Questions:

Code (non-working):


    m% char to make active
    m% code to execute for this character


    %\catcode`\+=12% reset catcode so we can use it (not needed with \OldPlus?)
    %% Perhaps this if-then construct should be a separate question?
    %% if binary addition operator
    %% if unary positive operator
    %%  \mathrel{\textcolor{red}{\OldPlus}}%



The ``math'' outside here is just to test that characters are active \emph{only} in math mode:

a=+b $c=+4+d+3!$ e=+f 

You have to be careful with the declarations if you load amsmath; in the body of the definition, use \std{<char>} where you need the old meaning.


  % #1 is the character, #2 is the definition
  \expandafter\edef\csname keep@#1@code\endcsname{\mathchar\the\mathcode`#1 }
  \AtBeginDocument{\mathcode`#1="8000 }%

\newcommand{\std}[1]{\csname keep@#1@code\endcsname}



a=+b $c=+4+d+3!-1$ e=+f


Don't do \let\OldPlus=+.

enter image description here

  • Why does the - require special handling? Are there other characters that require special processing? – Peter Grill Mar 19 '16 at 11:01
  • @PeterGrill I made a better fix. – egreg Mar 19 '16 at 11:14
  • You surely know that \expandafter\mathchardef\csname keep@#1@code\endcsname=\mathcode`#1 does similar work like your \edef. – wipet Mar 19 '16 at 13:43
  • @wipet Yes, thanks for reminding, though. – egreg Mar 19 '16 at 13:45
  • 1
    Due to \newmcodes@ having been recently changed in amsopn.sty to better behave with Unicode engines, this (the patch) won't work with XeLaTeX/LuaLaTeX. – user4686 May 13 '16 at 9:08

You can define \mathdef declarator and then use it:

\input opmac \localcolor

\def\mathdef#1{\mathcode`#1="8000 \bgroup \lccode`~=`#1\lowercase{\egroup\def~}}

\mathdef +{\mathbin{\Red\mathchar`+}}
\mathdef ={\mathrel{\Green\mathchar`=}}

aha + $a+b+c=d$


The \input opmac is here only for setting colors \Red, \Green etc. If you are using another macro for colors then use only the second line \def\mathdef... in your macros.

  • 1
    You surely know that \mathord{\mathchar`.} and \mathpunct{\mathchar`,} are different from . and , in math mode. Also \mathbin{\mathchar`-} is quite different from -. Period and comma are taken from the math letters font, whereas the minus is taken from the symbol font. – egreg Mar 19 '16 at 10:49
  • When I attempted to use this I got Illegal parameter number in definition of \mathdef. – Peter Grill Mar 19 '16 at 11:03

There are a few problems:

\MakeActiveChar does \def#1{#2} which is not permitted since #1 is a character that is not active. Here's an alternative with The \lowercase trick

   \catcode`#1=12 \mathcode`#1="8000 }

Apart from that, you have a missing = in a \let. The first equal is optional, so \let\a=\b is correct, the equal, however, is necessary when you want to let to = because if not, it would be treated as the optional one, so


should be


And the last part is that by making a character active in math mode, and then doing \newcommand\plus{\mathbin{\originalplus}} you get an endless loop like \def\a{\a}, so you need to use for instance \mathchar`\+ or \mathchar`+ instead.

Complete solution


   \catcode`#1=12 \mathcode`#1="8000 }




The ``math'' outside here is just to test that characters are active \emph{only} in math mode:

a=+b $c=+4+d+3!$ e=+f 

By the way, I defined \mathcolor because \textcolor works with \bgroup/\egroup which create an ord atom.

  • Can you post a compilable MWE? I get Missing \begin{document} when I replace my definition of \MakeActiveChar with yours. Also, can you explain why you suggest the double equal? If I use the double equal as you suggest, along with your ``\MakeActiveChar, I get TeX capacity exceeded, sorry [grouping levels=255]`. – Peter Grill Mar 19 '16 at 10:26
  • Okey, sorry, I just did it without trying, and the error comes from somewhere else. I'll edit. – Manuel Mar 19 '16 at 10:28
  • Still not a compilable MWE. And again I get LaTeX Error: Missing \begin{document} when I replace your \MakeActiveChar in the MWE I posted in the question. – Peter Grill Mar 19 '16 at 10:52
  • 1
    Sorry about the missing compilable example. The point about \std@minus in egreg answer is interesting, don't miss it. – Manuel Mar 19 '16 at 10:54

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