>>No. ([A-Za-z0-9]{1,20})-([A-Za-z0-9]{1,20}) matches between 1 and 20 alphanumeric characters followed by a single hyphen followed by an additional 1-20 alphanumeric characters. [A-Za-z0-9]{1,20} will match both the characters before and after the hyphen of the first expression as separate matches, but not the whole first expression. The second expression will also match where the hyphen is missing or there is only one set of 1-20 alphanumeric characters.
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>This is the part I cannot understand. We have a maximum of 20 characters. So, I do not understand why I received this mask if no more than 20 characters can be part of the field.

What are you trying to do with the expression? Based off of the original expression, I'm going to guess it is validation to ensure the entered text is alphanumeric with a single hyphen inserted somewhere.