Bash Regex needs attention

Question

I do not know that this is a duplicate question. I have, as I always do, research until I do not have a workable solution or have not figured it out by myself. I am not frivolous. I can split this string at the delimiter in minutes. That is not why I am here. I am security conscious. This is user input from command line input. I am trying to verify and validate user input and I do not consider that a wasted effort. I'm doing something fairly simple, however my level of expertise can't figure this out. I am taking a command line option input from a script and it goes into three variables.

$LOCAL_FILE
$DESTINATION_USER
$DESTINATION_FILE

I'll be using scp and /etc/hosts to verify before processing.

I take $DESTINATION_USER which is example: user@xu4-node1-2 and run the user thru my not working regex (Which I use elsewhere in an if conditional and it works fine); to verify format and remove the "user@" part so that I can awk /etc/hosts to get the IP of the user so that I can verify and put the hostname and ip into an associative array (That is used for a similar reason).

sUSER=$DESTINATION_USER
[[ "$sUSER" =~ "([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])(\.([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]{0,61}[a-zA-Z0-9]))*$" ]]
ip=$(awk '/^[[:space:]]*($|#)/{next}/'$sUSER'/{print $1; exit}' /etc/hosts)
declare -A dUSER[$ip]=$sUSER

For example, I need to verify user input "user@xu4-node1-2" and loose the "user@" part so all I have is "xu4-node1-2" so I can give it to the awk to give me the ip. My regex however gives me the whole "user@xu4-node1-2"

I use the regex in this:

elif [[ $i =~ ^([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])(\.([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]{0,61}[a-zA-Z0-9]))* ]]; then

and it works fine.

** Update

I was able to complete this code to resolve my problem. I am posting it here in case it can help anyone else. I included verification and validation of user input, un-tainting it and though it's not in this snippet, input from the hosts file as well. I also included some error handling.

    sUSER=$DESTINATION_USER
    sUSER="${sUSER#"${sUSER%%[![:space:]A-Za-z0-9._%+-]*}@"}"
    if [[ $sUSER =~ ^([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]{0,61}[a-zA-Z0-9])(\.([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\-]{0,61}[a-zA-Z0-9]))*$ ]]; then
      ip=$(awk '/^[[:space:]]*($|#)/{next}/'$sUSER'/{print $1; exit}' /etc/hosts)
      if ! [[ $ip ]]; then
        printf "\n${LtRED} --> Error - No reference IP address found in /etc/hosts file for $sUSER\n --> Make sure that your hosts file is properly formatted${RESET}\n\n"
        exit 1
      fi
      declare -A dUSER[$ip]=$sUSER
    else
      printf "\n${LtRED} --> Remote user address ( $sUSER ) is not formatted properly - Try Again${RESET}\n\n"
      exit 1
  fi