How to check if input string matches a specific format?

The lazy way is just to run

if [[ $mac_address == ??:??:??:??:??:?? ]]; then echo Heureka; fi

but this doesn't check whether it's a hex string. So if this is important

if [[ $mac_address =~ ^[0-9a-fA-F][0-9a-fA-F]:[0-9a-fA-F][0-9a-fA-F]:[0-9a-fA-F][0-9a-fA-F]:[0-9a-fA-F][0-9a-fA-F]:[0-9a-fA-F][0-9a-fA-F]:[0-9a-fA-F][0-9a-fA-F]$ ]]; then echo Heureka; fi

might be better. The later can be shortened to

if [[ $mac_address =~ ^([[:xdigit:]]{2}:){5}[[:xdigit:]]{2}$ ]]; then
    echo Heureka; 
fi

If the pattern matches I don't see a need to check for the correct length as well.

[[ $mac_address: =~ ^([[:xdigit:]]{2}:){6}$ ]]

I changed the name of the variable so that it consisted of legal characters, then used the =~ test operator to compare the value to the extended regular expression matching: at the beginning of the string ^, "2 hex digits and a colon" 5 times, followed by 2 hex digits, ending the string $:

#!/bin/bash

read -p "enter mac-address " mac_address

if [[ $mac_address =~ ^([[:xdigit:]][[:xdigit:]]:){5}[[:xdigit:]][[:xdigit:]]$ ]]
then
  echo good
else
  echo bad
fi