String replacement in latex - string

Question

I'd like to know how to replace parts of a string in latex. Specifically I'm given a measurement (like 3pt, 10mm, etc) and I'd like to remove the units of that measurement (so 3pt-->3, 10mm-->10, etc).
The reason why I'd like a command to do this is in the following piece of code:
\newsavebox{\mybox}
\sbox{\mybox}{Hello World!}
\newlength{\myboxw}
\newlength{\myboxh}
\settowidth{\myboxw}{\usebox{\mybox}}
\settoheight{\myboxh}{\usebox{\mybox}}
\begin{picture}(\myboxw,\myboxh)
\end{picture}
Basically I create a savebox called mybox. I insert the words "Hello World" into mybox. I create a new length/width, called myboxw/h. I then get the width/height of mybox, and store this in myboxw/h. Then I set up a picture environment whose dimensions correspond to myboxw/h. The trouble is that myboxw is returning something of the form "132.56pt", while the input to the picture environment has to be dimensionless: "\begin{picture}{132.56, 132.56}".
So, I need a command which will strip the units of measurement from a string.
Thanks.

Answer 1

Use the following trick:
{
\catcode`p=12 \catcode`t=12
\gdef\removedim#1pt{#1}
}
Then write:
\edef\myboxwnopt{\expandafter\removedim\the\myboxw}
\edef\myboxhnopt{\expandafter\removedim\the\myboxh}
\begin{picture}(\myboxwnopt,\myboxhnopt)
\end{picture}

Answer 2

Consider the xstring package at https://www.ctan.org/pkg/xstring.

Answer 3

The LaTeX kernel - latex.ltx - already provides \strip#pt, which you can use to strip away any reference to a length. Additionally, there's no need to create a length for the width and/or height of a box; \wd<box> returns the width, while \ht<box> returns the height:
\documentclass{article}
\makeatletter
\let\stripdim\strip#pt % User interface for \strip#pt
\makeatother
\begin{document}
\newsavebox{\mybox}
\savebox{\mybox}{Hello World!}
\begin{picture}(\stripdim\wd\mybox,\stripdim\ht\mybox)
\put(0,0){Hello world}
\end{picture}
\end{document}