Only accept 4 character long, decimal numbers as user input for read

I am writing a small script to easily capture weights, which are taken from user input with read -p.

I want all captured weights to be 4 characters long, decimal numbers in the format XX.X. For example 30.2 kg.

#!/bin/bash

# take input from prompt (-p) and store it in $varweight
read -p 'Enter weight in kg: ' varweight

How can I make sure users can only input decimal numbers in that format?

Thank you for your help.

Answer

One way would be to use a regex and assert the length to remove false positives (optionally). Of course the below would require the bash shell because the ~ is not POSIX compliant

str="30.4"
re='^[[:digit:]]{2}.[[:digit:]]$'

if [[ $str =~ $re ]]; then
    echo 'true'
fi

On a POSIX compliant shell though you could use case statement

case $str in  
   [0-9][0-9].[0-9]) echo good ;; 
   *) echo bad;; 
esac

A much better POSIX compliant tool would be to use expr to do the regex match.

if expr "$str" : '[[:digit:]]{2}.[[:digit:]]$' > /dev/null; then
    echo "true"
fi