backups: Allow remote repository usernames to start with numbers

Although useradd recommends starting with either a lowercase letter or an
underscore, there is nothing that consistently adheres to this recommendation
across systems. Because some systems do not follow this recommendation and this
recommendation is not a hard requirement, we should not prevent connections at
the validation stage.

Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/modules/backups/forms.py

@@ -95,7 +95,7 @@ def repository_validator(path):
     hostname = hostname.split('%')[0]
 
     # Validate username using Unix username regex
-    if not re.match(r'[a-z_][a-z0-9_-]*$', username):
+    if not re.match(r'[a-z0-9_][a-z0-9_-]*$', username):
         raise ValidationError(_(f'Invalid username: {username}'))
 
     # The hostname should either be a valid IP address or hostname

plinth/modules/backups/tests/test_validators.py

@@ -34,8 +34,10 @@ def test_repository_paths_validation():
 
 def test_repository_username_validation():
     """Test that usernames in repository string are validated properly."""
-    valid_usernames = ['sshuser', 'cypher_punk-2077', '_user', '_-_']
-    invalid_usernames = ['1two', 'somebody else']
+    valid_usernames = [
+        'sshuser', 'cypher_punk-2077', '_user', '_-_', '1two', '1234'
+    ]
+    invalid_usernames = ['somebody else']
     path_string = '{}@example.org:~/backups'
     _validate_repository(valid_usernames, invalid_usernames, path_string)