>When I first looked at the first one, I thought this meant alphanumeric characters + a dash character in all characters from 1 to 20 MINUS alphanumeric only in the first 12 characters. Which means, the dash would have been accepted only from position 13th and up.
>
>But, support gave me information that this means the dash is supported from position 2 and up.
>
>I really do not know what to believe now.

([A-Za-z0-9-]{1,20}) : at least one but less than 21 characters chosen from A to Z, a to z, 0 to 9 or the dash
- : the previous group MUST be followed by a dash (so yes, it means it is supported from position 2 and up to 21, not supported in position 22)
([A-Za-z0-9]{1,4}) : at least one but less than 5 characters chosen from A to Z, a to z, 0 to 9

So all of the following are valid:
-A-B
--B
ABC-DEF
ABC--DEF
AA--AB

Which I am not sure is what you want, I think you do not want the dash in the first expression?

NOTE: Position above refers to the relative position from the start of the matched string, if you want it to match only at START or/and END you must include ^ or/and $ for example

12345678901234567890--DEF will match even thou it has 22 characters before the mandatory dash, but your regex will match from the 2 on (2345678901234567890--DEF), to avoid this you need to start the regex with ^