The OR here is meant to indicate that there are two possible answers.
One of the participants in our workshop read this as a part of the regular expression. This mistake is very easy to make, as this lesson is also about regular expressions!
I suggest rewriting OR as 'or, alternatively' in lowercase.
In exercise “Finish the expression” in https://librarycarpentry.org/lc-data-intro/02-match-extract-strings/index.html the answer according to the lesson is:
[\w.-]+.[\w]{2,3} OR [\w.-]+.[\w]
The OR here is meant to indicate that there are two possible answers. One of the participants in our workshop read this as a part of the regular expression. This mistake is very easy to make, as this lesson is also about regular expressions!
I suggest rewriting OR as 'or, alternatively' in lowercase.