Do not allow slugs to start with a non-alphanumeric character

src/pretix/base/models/event.py

@@ -293,7 +293,7 @@ class Event(EventMixin, LoggedModel):
             "This will be used in URLs, order codes, invoice numbers, and bank transfer references."),
         validators=[
             RegexValidator(
-                regex="^[a-zA-Z0-9.-]+$",
+                regex="^[a-zA-Z0-9][a-zA-Z0-9.-]+$",
                 message=_("The slug may only contain letters, numbers, dots and dashes."),
             ),
             EventSlugBanlistValidator()