Bash Shell help regular expressions not working on password check - regex

Question

I am trying to make regular expression work with my password checker script but for some reason it is just not accepting it. What I am trying to do is require the following characters, the minimum of:
two upper case letters, two lower case letters, minimum 2 numbers and minimum two special characters. The special characters I am looking towards are just "!#" and that is it.
Here is the script,
#!/bin/bash
read -s -p "Enter Password: " password
password_length=${#password}
if [ $password_length -lt 8 -o $password_length -gt 20 ] ;then
echo -e "Invalid password - should be between 8 and 20 characters in length.";
echo ;
else
# Check for invalid characters
case $password in
*[^a-zA-Z0-9]* )
echo -e "Password contains invalid characters.";
echo ;
;;
* )
echo "Password accepted.";
echo;;
esac
fi

Answer 1

I don't have a good answer. At the moment I'm playing around with using grep as it has a far more powerful regex engine.
Something like this:
#!/usr/bin/env bash
while true; do
echo "Password must contain 2 upper case characters and two numbers"
read -s -p "Enter Password: " password
echo
FAIL=no
# Mininum 8 characters
[[ ${#password} -ge 8 ]] || FAIL=yes
# Must contain 2 upper case letters
echo $password | grep -q "[A-Z].*[A-Z]" || FAIL=yes
# Must contain 2 digits
echo $password | grep -q "[0-9].*[0-9]" || FAIL=yes
# Must contain one non-alphanumeric character (no spaces)
echo $password | grep -q "[^a-zA-Z0-9 ]" || FAIL=yes
[[ ${FAIL} == "no" ]] && break
echo "Password invalid"
done
echo "Password $password is valid"
Anyway, what I would do is to test the possible password against each requirement separately, as it's easy to get lost in regular expressions, and you might miss some edge cases.

Answer 2

BASH regex engine doesn't support lookarounds as in your regex.
This example is for :
Password should be at least 6 characters long with at least one digit and at least one Upper case Alphabet.
You can use following shell glob check to make sure password matches your criteria:
[[ ${#s} -ge 6 && "$s" == [A-Z] && "$s" == [a-z] && "$s" == [0-9] ]]
ref : Regex password validation in Bash shell