Index: cfe/trunk/lib/Format/FormatTokenLexer.cpp
===================================================================
--- cfe/trunk/lib/Format/FormatTokenLexer.cpp
+++ cfe/trunk/lib/Format/FormatTokenLexer.cpp
@@ -157,7 +157,9 @@
   // postfix unary operators. If the '++' is followed by a non-operand
   // introducing token, the slash here is the operand and not the start of a
   // regex.
-  if (Prev->isOneOf(tok::plusplus, tok::minusminus))
+  // `!` is an unary prefix operator, but also a post-fix operator that casts
+  // away nullability, so the same check applies.
+  if (Prev->isOneOf(tok::plusplus, tok::minusminus, tok::exclaim))
     return (Tokens.size() < 3 || precedesOperand(Tokens[Tokens.size() - 3]));
 
   // The previous token must introduce an operand location where regex
Index: cfe/trunk/unittests/Format/FormatTestJS.cpp
===================================================================
--- cfe/trunk/unittests/Format/FormatTestJS.cpp
+++ cfe/trunk/unittests/Format/FormatTestJS.cpp
@@ -941,6 +941,7 @@
   verifyFormat("var x = a ? /abc/ : /abc/;");
   verifyFormat("for (var i = 0; /abc/.test(s[i]); i++) {\n}");
   verifyFormat("var x = !/abc/.test(y);");
+  verifyFormat("var x = foo()! / 10;");
   verifyFormat("var x = a && /abc/.test(y);");
   verifyFormat("var x = a || /abc/.test(y);");
   verifyFormat("var x = a + /abc/.search(y);");