Yhyjyk

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:

documentclassarticle
usepackage[utf8]inputenc

newcommandcmd[1]#1 % change here to capture only the number. 
begindocument
 cmd12pt % print 12pt while I would get only 12, in a generic case.
enddocument

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 argXX55km is 55, and the output of cmdkm is blank (empty).

% !TEX TS-program = lualatex
documentclassarticle
usepackageluacode % for 'luacode' environment
beginluacode
function get_num ( s )
 tex.sprint ( ( s:gsub ( "([%d%.%,%-%+]*)([%a%s]*)" , "%1" ) ) )
end
endluacode
newcommandcmd[1]directluaget_num("#1") % "wrapper" macro

begindocument
cmd12pt, $cmd-47km$, cmd+5.7in, cmd3,14159CM
enddocument

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

documentclassarticle
usepackagexparse

ExplSyntaxOn

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

ExplSyntaxOff

begindocument

getnumber12pt, $getnumber-47km$, getnumber+5.7in, getnumber3,14159CM

enddocument

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

documentclassarticle
usepackagepgf

newcommandcmd[1]pgfmathparse#1pgfmathresult 
begindocument
 cmd12pt cmd1cm 
enddocument

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.