hosts.py: Generalized supported operations of group pattern

hosts.py

@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 
 import json
+import re
 import sys
 import yaml
 
@@ -117,6 +118,12 @@ def _read_yaml(path):
         except yaml.YAMLError as e:
             return AnsibleError(e)
 
+GROUPS_PATTERN_OPS = {
+    "": lambda old, add: old | add,
+    "!": lambda old, add: old - add,
+}
+GROUPS_PATTERN_OPS_NAMES = "".join(GROUPS_PATTERN_OPS.keys())
+GROUPS_PATTERN = re.compile(r'^(?P<operation>[' + GROUPS_PATTERN_OPS_NAMES + r']?)(?P<group_name>[^' + GROUPS_PATTERN_OPS_NAMES + r'].*)$')
 def _parse_group_aliasses(inv, data):
     for group, syntax in data.items():
         if isinstance(syntax, str):
@@ -131,10 +138,10 @@ def _parse_group_aliasses(inv, data):
             group_list.insert(0, 'all') # remove group from all for inversion
         hosts = set()
         for group_name in group_list:
-            if group_name[0] == '!':
-                hosts -= inv.all_hosts_of_group(group_name[1:])
-            else:
-                hosts |= inv.all_hosts_of_group(group_name)
+            group_matched = GROUPS_PATTERN.match(group_name)
+            add = inv.all_hosts_of_group(group_matched.group('group_name'))
+            op = GROUPS_PATTERN_OPS[group_matched.group('operation')]
+            hosts = op(hosts, add)
         inv.add_hosts_to_group(hosts, group)
 
 def _parse_groups(inv, data):