order of (escaped) characters in regex range

Dear List,

I found this interesting:

SELECT regexp_matches('123-A' , E'(3[A-Z\- ])');
ERROR:  invalid regular expression: invalid character range

whereas:
SELECT regexp_matches('123-A' , E'(3[\- A-Z])');
 regexp_matches
----------------
 {3-}
(1 row)

Notice the order of (escaped) characters and ranges in the last bit of the expression.

Am I missing some key concept of the regular expression?

Regards,
Rob