Fix default rse attribute reg-exp
Motivation
----------
The `default_rse_attribute` regular expression was made hard-coded (instead of pulled in from the schema) based on concerns that the expression parser could be impacted by faulty attributes. However, the hard-coded regexp was defined to strict, causing issues for Belle II.

Modification
------------
Change regexp to `([A-Za-z0-9]+([_-][A-Za-z0-9]+)*)`

diff --git a/lib/rucio/core/rse_expression_parser.py b/lib/rucio/core/rse_expression_parser.py
index de3a97d86..6eb86345f 100644
--- a/lib/rucio/core/rse_expression_parser.py
+++ b/lib/rucio/core/rse_expression_parser.py
@@ -39,7 +39,7 @@ from rucio.core.rse import list_rses, get_rses_with_attribute, get_rse_attribute
 from rucio.db.sqla.session import transactional_session
 
 
-DEFAULT_RSE_ATTRIBUTE = r'([A-Z0-9]+([_-][A-Za-z0-9]+)*)'
+DEFAULT_RSE_ATTRIBUTE = r'([A-Za-z0-9]+([_-][A-Za-z0-9]+)*)'
 RSE_ATTRIBUTE = r'([A-Za-z0-9\\._-]+[=<>][A-Za-z0-9_-]+)'
 PRIMITIVE = r'(\\(*(%s|%s|%s)\\)*)' % (RSE_ATTRIBUTE, DEFAULT_RSE_ATTRIBUTE, r'\\*')
 UNION = r'(\\|%s)' % (PRIMITIVE)
['lib/rucio/core/rse_expression_parser.py']