order of (escaped) characters in regex range

[InterRob <rob.marjot@xxxxxxxxx>]

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