diff --git a/services/web/frontend/js/features/source-editor/lezer-latex/latex.grammar b/services/web/frontend/js/features/source-editor/lezer-latex/latex.grammar
index 9ed0073b97..82efa49c4a 100644
--- a/services/web/frontend/js/features/source-editor/lezer-latex/latex.grammar
+++ b/services/web/frontend/js/features/source-editor/lezer-latex/latex.grammar
@@ -205,7 +205,7 @@ BareFilePathArgument {
 }
 
 DefinitionArgument {
-    !argument NewLine? Whitespace* OpenBrace DefinitionFragment CloseBrace
+    !argument NewLine? Whitespace* OpenBrace DefinitionFragment? CloseBrace
 }
 
 MacroParameter {
@@ -481,12 +481,10 @@ TikzPictureContent {  /// same as Text but with added allowed characters
 }
 
 DefinitionFragment {
-    ( KnownCommand<TextArgument>
-    | CtrlSeq optionalWhitespace?
-    | CtrlSym
+    ( DefinitionFragmentCommand
     | Begin
     | End
-    | NonEmptyGroup<DefinitionFragment>
+    | Group<DefinitionFragment>
     | Dollar
     | OpenParenCtrlSym
     | CloseParenCtrlSym
@@ -494,7 +492,6 @@ DefinitionFragment {
     | CloseBracketCtrlSym
     | LeftCtrlSeq
     | RightCtrlSeq
-    | KnownCtrlSym
     | BlankLine
     | NewLine
     | Normal
@@ -515,8 +512,17 @@ DefinitionFragment {
         ParagraphCtrlSeq |
         SubParagraphCtrlSeq
       >
-    )*
+    )+
+}
 
+DefinitionFragmentArgument {
+    OpenBrace DefinitionFragment? CloseBrace
+}
+
+DefinitionFragmentCommand {
+      KnownCommand<TextArgument>
+    | UnknownCommand { genericUnknownCommandWithOptionalArguments<DefinitionFragmentArgument, OptionalArgument> }
+    | KnownCtrlSym
 }
 
 KnownEnvironment {
@@ -690,7 +696,7 @@ Math {
 
 MathCommand {
     KnownCommand<MathArgument>
-  | MathUnknownCommand
+  | MathUnknownCommand { genericUnknownCommand<MathArgument> }
   | KnownCtrlSym
 }
 
@@ -699,8 +705,18 @@ MathCommand {
     endOfArguments
 }
 
-MathUnknownCommand {
-    CtrlSeq (hasMoreArguments optionalWhitespace? MathArgument)* endOfArguments
+@external tokens argumentListWithOptionalTokenizer from "./tokens.mjs" {
+    hasMoreArgumentsOrOptionals,
+    endOfArgumentsAndOptionals
+}
+
+genericUnknownCommand<ArgumentType> {
+    CtrlSeq (hasMoreArguments optionalWhitespace? ArgumentType)* endOfArguments
+  | CtrlSym
+}
+
+genericUnknownCommandWithOptionalArguments<ArgumentType, OptionalArgumentType> {
+    CtrlSeq (hasMoreArgumentsOrOptionals optionalWhitespace? (ArgumentType | OptionalArgumentType))* endOfArgumentsAndOptionals
   | CtrlSym
 }
 
diff --git a/services/web/frontend/js/features/source-editor/lezer-latex/tokens.mjs b/services/web/frontend/js/features/source-editor/lezer-latex/tokens.mjs
index 25ae9d83fd..5b1cfff016 100644
--- a/services/web/frontend/js/features/source-editor/lezer-latex/tokens.mjs
+++ b/services/web/frontend/js/features/source-editor/lezer-latex/tokens.mjs
@@ -82,6 +82,8 @@ import {
   // Marker for end of argument lists
   endOfArguments,
   hasMoreArguments,
+  hasMoreArgumentsOrOptionals,
+  endOfArgumentsAndOptionals,
 } from './latex.terms.mjs'
 
 const MAX_ARGUMENT_LOOKAHEAD = 100
@@ -254,29 +256,45 @@ function _char(s) {
 
 const CHAR_BACKSLASH = _char('\\')
 const CHAR_OPEN_BRACE = _char('{')
+const CHAR_OPEN_BRACKET = _char('[')
 const CHAR_CLOSE_BRACE = _char('}')
 const CHAR_TAB = _char('\t')
 const CHAR_SPACE = _char(' ')
 const CHAR_NEWLINE = _char('\n')
 
-export const argumentListTokenizer = new ExternalTokenizer(
-  input => {
-    for (let i = 0; i < MAX_ARGUMENT_LOOKAHEAD; ++i) {
-      const next = input.peek(i)
-      if (next === CHAR_SPACE || next === CHAR_TAB) {
-        continue
+const lookaheadTokenizer = getToken =>
+  new ExternalTokenizer(
+    input => {
+      for (let i = 0; i < MAX_ARGUMENT_LOOKAHEAD; ++i) {
+        const next = input.peek(i)
+        if (next === CHAR_SPACE || next === CHAR_TAB) {
+          continue
+        }
+        const token = getToken(next)
+        if (token) {
+          input.acceptToken(token)
+          return
+        }
       }
-      if (next === CHAR_OPEN_BRACE) {
-        input.acceptToken(hasMoreArguments)
-        return
-      } else {
-        input.acceptToken(endOfArguments)
-        return
-      }
-    }
-  },
-  { contextual: false, fallback: true }
-)
+    },
+    { contextual: false, fallback: true }
+  )
+
+export const argumentListTokenizer = lookaheadTokenizer(next => {
+  if (next === CHAR_OPEN_BRACE) {
+    return hasMoreArguments
+  } else {
+    return endOfArguments
+  }
+})
+
+export const argumentListWithOptionalTokenizer = lookaheadTokenizer(next => {
+  if (next === CHAR_OPEN_BRACE || next === CHAR_OPEN_BRACKET) {
+    return hasMoreArgumentsOrOptionals
+  } else {
+    return endOfArgumentsAndOptionals
+  }
+})
 
 const CHAR_AT_SYMBOL = _char('@')