Supply completions for negative regex categories as well.

src/EditorFeatures/Test2/IntelliSense/CSharpCompletionCommandHandlerTests_Regex.vb

@@ -271,6 +271,30 @@ class c
             End Using
         End Function
 
+        <WpfTheory, CombinatorialData, Trait(Traits.Feature, Traits.Features.Completion)>
+        Public Async Function TestNegativeCategory(showCompletionInArgumentLists As Boolean) As Task
+            Using state = TestStateFactory.CreateCSharpTestState(
+                <Document><![CDATA[
+using System.Text.RegularExpressions;
+class c
+{
+    void goo()
+    {
+        var r = new Regex(@"\P$$");
+    }
+}
+]]></Document>, showCompletionInArgumentLists:=showCompletionInArgumentLists)
+
+                state.SendTypeChars("{")
+                Await state.AssertCompletionSession()
+
+                Assert.True(state.GetCompletionItems().Any(Function(i) i.DisplayText = "IsGreek"))
+
+                state.SendTab()
+                Await state.AssertNoCompletionSession()
+            End Using
+        End Function
+
         <WpfTheory, CombinatorialData, Trait(Traits.Feature, Traits.Features.Completion)>
         Public Async Function NotInInterpolatedString(showCompletionInArgumentLists As Boolean) As Task
             Using state = TestStateFactory.CreateCSharpTestState(

src/Features/Core/Portable/EmbeddedLanguages/RegularExpressions/RegexEmbeddedCompletionProvider.cs

@@ -259,19 +259,16 @@ private static void ProvideTopLevelCompletions(EmbeddedCompletionContext context
 
             var index = tree.Text.IndexOf(previousVirtualChar);
             if (index >= 2 &&
-                tree.Text[index - 2] == '\\' &&
-                tree.Text[index - 1] == 'p')
+                tree.Text[index - 2] == '\\')
             {
-                var slashChar = tree.Text[index - 1];
-                var result = FindToken(tree.Root, slashChar);
-                if (result == null)
+                var escapeChar = tree.Text[index - 1];
+                if (escapeChar == 'p' || escapeChar == 'P')
                 {
+                    var result = FindToken(tree.Root, escapeChar);
+                    if (result == null)
                         return;
-                }
 
-                var (parent, _) = result.Value;
-                if (parent is RegexEscapeNode)
-                {
+                    if (result.Value.parent is RegexEscapeNode)
                         ProvideEscapeCategoryCompletions(context);
                 }
             }