Document realm filter regexp oddities

(imported from commit 987d8ea679f40c31f2fe2280b6d585f82b20e606)

zerver/lib/actions.py

@@ -2284,6 +2284,10 @@ def notify_realm_filters(realm):
     notice = dict(event=dict(type="realm_filters", realm_filters=realm_filters), users=user_ids)
     tornado_callbacks.send_notification(notice)
 
+# NOTE: Regexes must be simple enough that they can be easily translated to JavaScript
+# RegExp syntax. In addition to JS-compatible syntax, the following features are available:
+#   * Named groups will be converted to numbered groups automatically
+#   * Inline-regex flags will be stripped, and where possible translated to RegExp-wide flags
 def do_add_realm_filter(realm, pattern, url_format_string):
     RealmFilter(realm=realm, pattern=pattern,
                 url_format_string=url_format_string).save()

zerver/management/commands/realm_filters.py

@@ -11,6 +11,12 @@ class Command(BaseCommand):
 
 Usage: python manage.py realm_filters foo.com PATTERN URLPATTERN
 
+NOTE: Regexes must be simple enough that they can be easily translated to JavaScript
+      RegExp syntax. In addition to JS-compatible syntax, the following features are available:
+
+      * Named groups will be converted to numbered groups automatically
+      * Inline-regex flags will be stripped, and where possible translated to RegExp-wide flags
+
 Example: python manage.py realm_filters --realm=zulip.com --op=add '#(?P<id>[0-9]{2,8})' 'https://trac.humbughq.com/ticket/%(id)s'
 Example: python manage.py realm_filters --realm=zulip.com --op=remove '#(?P<id>[0-9]{2,8})'
 Example: python manage.py realm_filters --realm=zulip.com --op=show