SHELL

if regex bash

#ensure there are no quotes arround the regex
if [[ "${DOMAINS}" =~ *..+ ]]; then ...

bash regex if condition

Use following structure:
if [[ $digit =~ [0-9] ]]; then //run if a digit included in $digit string
    echo "$digit is a digit"
else
    echo "oops"
fi

bash if else regex

#!/bin/bash

if [[ "$date" =~ ^[0-9]{8}$ ]]; then
    echo "Valid date"
else
    echo "Invalid date"
fi