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Let us say that I have a function, in which I give a number plus a unit. I would like to get only the number, is it possible ?

Here is a MWE:

documentclass{article}

usepackage[utf8]{inputenc}



newcommand{cmd}[1]{#1} % change here to capture only the number.  

begin{document}

    cmd{12pt} % print 12pt while I would get only 12, in a generic case.

end{document}

Here's a LuaLaTeX-based solution. It sets up a LaTeX macro called cmd -- a "wrapper" -- that invokes a Lua function that does all of the work. The Lua function expects its argument to consist of two parts: the first part is numeric, i.e., consists of the digits 0 thru 9, plus possibly the characters ,, ., -, and +; the part second is alphabetic, i.e., uppercase and lowercase letters, plus possibly whitespace.

Per your typesetting objective, the function returns just the numeric, part. If the argument of cmd does not start with a numeric component, the prefix part is discarded as well. E.g., the output of arg{XX55km} is 55, and the output of cmd{km} is blank (empty).

% !TEX TS-program = lualatex

documentclass{article}

usepackage{luacode} % for 'luacode' environment

begin{luacode}

function get_num ( s )

  tex.sprint ( ( s:gsub ( "([%d%.%,%-%+]*)([%a%s]*)" , "%1" ) ) )

end

end{luacode}

newcommandcmd[1]{directlua{get_num("#1")}}  % "wrapper" macro



begin{document}

cmd{12pt}, $cmd{-47km}$, cmd{+5.7in}, cmd{3,14159CM}

end{document}

Assuming the unit consists of two characters, you can do it in an expandable way:

documentclass{article}

usepackage{xparse}



ExplSyntaxOn



NewExpandableDocumentCommand{getnumber}{m}

 {

  tl_range:nnn { #1 } { 1 } { -3 } % from the first to the last but two character

 }



ExplSyntaxOff



begin{document}



getnumber{12pt}, $getnumber{-47km}$, getnumber{+5.7in}, getnumber{3,14159CM}



end{document}

pgf does that without the need to invoke external programs and converts the units into points.

documentclass{article}

usepackage{pgf}



newcommand{cmd}[1]{pgfmathparse{#1}pgfmathresult} 

begin{document}

    cmd{12pt} cmd{1cm} 

end{document}

Note that if you're bugged by the .0: this can easily be removed with pgfmathprintnumber[<your number format here>]{pgfmathresult} if you choose a number format that you like.