adb shell regular expression doesn't work as tested locally

Question

First, sorry if my question is obscure or in an inconvenient format. This is my first post here :D.

My issue is that I have a script, let's say test.sh which reads an input, and validates if it's a positive integer (reg ex used from this post: BASH: Test whether string is valid as an integer?):

#!/bin/sh
echo -n " enter number <"
read num

if [[ $num =~ ^-?[0-9]+$ ]]     #if num contains any symbols/letters 
then                            # anywhere in the string
  echo "not a positive int"
  exit
else
  echo "positive int read"
fi

I am running this script on my android device (Xiaomi Mi3 w) using adb shell and the error: syntax error: =~ unexpected operator keeps displaying.

First, is my regex even correct? Second, any hints on how I can overcome this syntax error?

Answer 1

I had to use ksh expression as shown below to get this to work.

case $num in
    +([0-9])*(.)*([0-9]) )
          # Variable positive integer
      echo "positive integer"
          ;;
    *) 
          # Not a positive integer
      echo "NOPE"
      exit
          ;;
esac