Is there a way to autoreplace .../... with \frac{...}{...}?

For example, 143/23 should be rendered as \frac{143}{23}

  • It is possible (but fragile) to do this sort of replacement in TeX in simple cases (see the supplied answer) but it is really much better to use your text editor to do the replacement in the document source. Oct 5, 2015 at 11:51
  • The main problem is, how is one supposed to "know" where the numerator starts and where the denominator ends? Is it safe to assume that the numerator and the denominator will always consist purely of digits and that there will be no nested fraction terms?
    – Mico
    Oct 5, 2015 at 11:58
  • @Mico Yes, there are one digit in numerator and one in denumerator.
    – Simankov
    Oct 5, 2015 at 12:01
  • One might also ask why you're doing this. In inline text, it's very often preferable to have slashed fractions over stacked fractions. Even $1 / 2$ is easier to read than $\frac{1}{2}$, not even to mention such "complicated" cases as $\frac{x^2}{2}$.
    – wchargin
    Oct 5, 2015 at 18:30
  • @WChargin To be honest, in a table I used $1 / 2$ because of $frac{}{}$ and $dfrac{}{}$ version very ugly in table.
    – Simankov
    Oct 5, 2015 at 18:44

2 Answers 2


Here's a LuaLaTeX-based solution. It works for all fractional expressions where both the numerator and the denominator contain only (one or more) digits, and it works both in inline-math and display-math modes. In order for \frac to work, the fractional terms must be in math-mode material. Only fractional expressions with digits are processed; expressions such as a/b will not be modified.

If you want "large" fractions throughout the document, be sure to load the amsmath package and change frac to dfrac in the Lua function below.

In case you're curious: The lua function replace_slash uses so-called captures to identify the numerator and denominator terms. The lua function is assigned to the process_input_buffer callback. This means that the function operates on the input before TeX starts its own processing. Thus, TeX's "eyes" will never even see an expression such as $1/2$; instead, they'll only get to see the expression $\frac{1}{2}$.

(Addendum 9 Nov 2015: Generalized the Lua code so that (i) signed integers (positive or negative) and (ii) any whitespace in the input are handled correctly.)

enter image description here

% !TEX TS-program = lualatex


function replace_slash ( line )
   return string.gsub ( line, "([%+%-]?%d+)%s-/%s-([%+%-]?%d+)", "\\frac{%1}{%2}" )
luatexbase.add_to_callback ( "process_input_buffer", replace_slash , "replace_slash" )


How do we know that $55/110=1/2$?

$\displaystyle 55/110 = 1/2 $

Not modified: $a/(b+c)$, $x/y$, $2/x$

Addendum: Allowing an optional factorial symbol, viz., !, in the numerator and/or denominator would be very easy: In the Lua function, simply replace the search string




In Lua's pattern matching jargon, %s- means "0 or more occurrences of whitespace", and the !? substring means "0 or 1 instance of !". With this modification, $3! / 2!$ will be typeset as $\frac{3!}{2!}$.

  • @Mico luacode is still needed (or you could use \directlua but have to do something about %) My comment was perhaps a bit garbled, but anyway I'll remove both comments as the question is now edited Oct 5, 2015 at 13:23

There is TeX primitive \over. \frac is not the same, but the results in the basic case will be the same. Hence you can write


There are some disadvantages of making / an active character, but it is your choice.

MWE (please make them in the future) in your situation:




  • Yes, I immediately get disadvantage. It's crash). Thank you anyway
    – Simankov
    Oct 5, 2015 at 11:40
  • @Simankov Hence we ask for MWE, where the real case can be tested. In particular, if you have / in other meaning than division, you should expect problem. But my MWE is working for you? Oct 5, 2015 at 11:44
  • Yes, I wrap all my fractions in $$, and it is working now. But sizes of these fractions aren't good. They are so small compared to integer elements. Is there a way to fix it? I saw that dfrac{}{} looks better
    – Simankov
    Oct 5, 2015 at 11:49
  • @Simankov If you are using amstex, as I can guess, I will recommend you rewritng you file and replacing #1/#2 by \frac{#1}{#2} manually. Oct 5, 2015 at 11:56
  • 1
    Okay. Really, I just must RTFM.I am like a monkey in latex now.
    – Simankov
    Oct 5, 2015 at 11:59

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