diff --git a/src/Lean/Elab/Syntax.lean b/src/Lean/Elab/Syntax.lean
index 953ddc79ef..e2720bdcd9 100644
--- a/src/Lean/Elab/Syntax.lean
+++ b/src/Lean/Elab/Syntax.lean
@@ -231,9 +231,16 @@ private def declareSyntaxCatQuotParser (catName : Name) : CommandElabM Unit := d
 
 @[builtinCommandElab syntaxCat] def elabDeclareSyntaxCat : CommandElab := fun stx => do
   let catName  := stx[1].getId
+  let catBehavior :=
+    if stx[2].isNone then
+      Parser.LeadingIdentBehavior.default
+    else if stx[2][3].getKind == ``Parser.Command.catBehaviorBoth then
+      Parser.LeadingIdentBehavior.both
+    else
+      Parser.LeadingIdentBehavior.symbol
   let attrName := catName.appendAfter "Parser"
   let env ← getEnv
-  let env ← liftIO $ Parser.registerParserCategory env attrName catName
+  let env ← Parser.registerParserCategory env attrName catName catBehavior
   setEnv env
   declareSyntaxCatQuotParser catName
 
diff --git a/src/Lean/Parser/Basic.lean b/src/Lean/Parser/Basic.lean
index 47ee936c8a..fdc301496a 100644
--- a/src/Lean/Parser/Basic.lean
+++ b/src/Lean/Parser/Basic.lean
@@ -1593,8 +1593,7 @@ inductive LeadingIdentBehavior where
   | default
   | symbol
   | both
-  deriving Inhabited, BEq
-
+  deriving Inhabited, BEq, Repr
 
 /--
   Each parser category is implemented using a Pratt's parser.
diff --git a/src/Lean/Parser/Syntax.lean b/src/Lean/Parser/Syntax.lean
index 50eeebf5f1..1d8750f9e2 100644
--- a/src/Lean/Parser/Syntax.lean
+++ b/src/Lean/Parser/Syntax.lean
@@ -71,7 +71,10 @@ def notationItem := ppSpace >> withAntiquot (mkAntiquot "notationItem" `Lean.Par
 @[builtinCommandParser] def «macro_rules» := suppressInsideQuot (leading_parser optional docComment >> Term.attrKind >> "macro_rules" >>  optKind >> Term.matchAlts)
 @[builtinCommandParser] def «syntax»      := leading_parser optional docComment >> Term.attrKind >> "syntax " >> optPrecedence >> optNamedName >> optNamedPrio >> many1 (syntaxParser argPrec) >> " : " >> ident
 @[builtinCommandParser] def syntaxAbbrev  := leading_parser "syntax " >> ident >> " := " >> many1 syntaxParser
-@[builtinCommandParser] def syntaxCat     := leading_parser "declare_syntax_cat " >> ident
+def catBehaviorBoth   := leading_parser nonReservedSymbol "both"
+def catBehaviorSymbol := leading_parser nonReservedSymbol "symbol"
+def catBehavior := optional ("(" >> nonReservedSymbol "behavior" >> " := " >> (catBehaviorBoth <|> catBehaviorSymbol) >> ")")
+@[builtinCommandParser] def syntaxCat := leading_parser "declare_syntax_cat " >> ident >> catBehavior
 def macroArg  := leading_parser optional (atomic (ident >> checkNoWsBefore "no space before ':'" >> ":")) >> syntaxParser argPrec
 def macroRhs (quotP : Parser) : Parser := leading_parser "`(" >> incQuotDepth quotP >> ")" <|> termParser
 def macroTailTactic   : Parser := atomic (" : " >> identEq "tactic") >> darrow >> macroRhs Tactic.seq1