Do you have any details on what you've attempted so far? Taking a look into it from my end, when I test using Regex101.com I am able to use the following:
Regular expression:
^website.com$
Test String:
website.com
help.website.com
In the above, example, we will only match the 1st string as we're stating that the string should start with website and end with .com, so if any result shows help. etc, this will be excluded.