#!/bin/bash

users=("admin" "root1" "John_Doe" "Alice" "Bob42")

badChar=false

for username in "${users[@]}"; do

    badChar=false

    for ((i=0; i<${#username}; i++)); do
        char="${username:i:1}"
        if ! [[ "$char" =~ [A-Za-z] ]]; then
            badChar=true
            break
        fi
    done

    firstChar="${username:0:1}"
    validFirstLetter=false

    if [[ "$firstChar" =~ [A-Za-z] ]]; then
        validFirstLetter=true
    fi

    if ! $badChar && (( ${#username} <= 20 )) && $validFirstLetter; then
        echo "The username $username is valid"
    else
        echo "The username $username is not valid"
    fi

done