diff --git a/services/web/frontend/js/features/source-editor/lezer-bibtex/bibtex.grammar b/services/web/frontend/js/features/source-editor/lezer-bibtex/bibtex.grammar
index 1fa519d4bc..caafb570ee 100644
--- a/services/web/frontend/js/features/source-editor/lezer-bibtex/bibtex.grammar
+++ b/services/web/frontend/js/features/source-editor/lezer-bibtex/bibtex.grammar
@@ -7,7 +7,7 @@
   whiteSpace { $[\t\n ]+ }
   Identifier { ![,{} \t\n%]+ }
   StringName { $[a-zA-Z:_] $[a-zA-Z:_0-9-]* }
-  FieldName {$[a-zA-Z-_0-9]+}
+  FieldName {$[a-zA-Z-_0-9+]+}
   LiteralString {
     '"' (!["] | "\\" _)* '"'?
   }