Add tests that validate a bunch of different pattern detection codepaths.

src/EditorFeatures/CSharpTest/Classification/SemanticClassifierTests.cs

@@ -2745,6 +2745,162 @@ void Goo()
 Regex.Anchor("^"));
         }
 
+        [WpfFact, Trait(Traits.Feature, Traits.Features.Classification)]
+        public async Task TestRegex4()
+        {
+            await TestAsync(
+@"
+using System.Text.RegularExpressions;
+
+class Program
+{
+    void Goo()
+    {
+        var r = /* lang=regex */@""$\a(?#comment)"";
+    }
+}",
+Keyword("var"),
+Regex.Anchor("$"),
+Regex.OtherEscape("\\"),
+Regex.OtherEscape("a"),
+Regex.Comment("(?#comment)"));
+        }
+
+        [WpfFact, Trait(Traits.Feature, Traits.Features.Classification)]
+        public async Task TestRegex5()
+        {
+            await TestAsync(
+@"
+using System.Text.RegularExpressions;
+
+class Program
+{
+    void Goo()
+    {
+        var r = /* lang=regexp */@""$\a(?#comment)"";
+    }
+}",
+Keyword("var"),
+Regex.Anchor("$"),
+Regex.OtherEscape("\\"),
+Regex.OtherEscape("a"),
+Regex.Comment("(?#comment)"));
+        }
+
+        [WpfFact, Trait(Traits.Feature, Traits.Features.Classification)]
+        public async Task TestRegex6()
+        {
+            await TestAsync(
+@"
+using System.Text.RegularExpressions;
+
+class Program
+{
+    void Goo()
+    {
+        var r = /* lang=regexp */@""$\a(?#comment) # not end of line comment"";
+    }
+}",
+Keyword("var"),
+Regex.Anchor("$"),
+Regex.OtherEscape("\\"),
+Regex.OtherEscape("a"),
+Regex.Comment("(?#comment)"),
+Regex.Text(" # not end of line comment"));
+        }
+
+        [WpfFact, Trait(Traits.Feature, Traits.Features.Classification)]
+        public async Task TestRegex7()
+        {
+            await TestAsync(
+@"
+using System.Text.RegularExpressions;
+
+class Program
+{
+    void Goo()
+    {
+        var r = /* lang=regexp,ignorepatternwhitespace */@""$\a(?#comment) # is end of line comment"";
+    }
+}",
+Keyword("var"),
+Regex.Anchor("$"),
+Regex.OtherEscape("\\"),
+Regex.OtherEscape("a"),
+Regex.Comment("(?#comment)"),
+Regex.Comment("# is end of line comment"));
+        }
+
+        [WpfFact, Trait(Traits.Feature, Traits.Features.Classification)]
+        public async Task TestRegex8()
+        {
+            await TestAsync(
+@"
+using System.Text.RegularExpressions;
+
+class Program
+{
+    void Goo()
+    {
+        var r = /* lang = regexp , ignorepatternwhitespace */@""$\a(?#comment) # is end of line comment"";
+    }
+}",
+Keyword("var"),
+Regex.Anchor("$"),
+Regex.OtherEscape("\\"),
+Regex.OtherEscape("a"),
+Regex.Comment("(?#comment)"),
+Regex.Comment("# is end of line comment"));
+        }
+
+        [WpfFact, Trait(Traits.Feature, Traits.Features.Classification)]
+        public async Task TestRegex9()
+        {
+            await TestAsync(
+@"
+using System.Text.RegularExpressions;
+
+class Program
+{
+    void Goo()
+    {
+        var r = new Regex(@""$\a(?#comment) # is end of line comment"", RegexOptions.IgnorePatternWhitespace);
+    }
+}",
+Keyword("var"),
+Class("Regex"),
+Regex.Anchor("$"),
+Regex.OtherEscape("\\"),
+Regex.OtherEscape("a"),
+Regex.Comment("(?#comment)"),
+Regex.Comment("# is end of line comment"),
+Enum("RegexOptions"),
+EnumMember("IgnorePatternWhitespace"));
+        }
+
+        [WpfFact, Trait(Traits.Feature, Traits.Features.Classification)]
+        public async Task TestRegex10()
+        {
+            await TestAsync(
+@"
+using System.Text.RegularExpressions;
+
+class Program
+{
+    void Goo()
+    {
+        var r = new Regex(@""$\a(?#comment) # is not end of line comment"");
+    }
+}",
+Keyword("var"),
+Class("Regex"),
+Regex.Anchor("$"),
+Regex.OtherEscape("\\"),
+Regex.OtherEscape("a"),
+Regex.Comment("(?#comment)"),
+Regex.Text(" # is not end of line comment"));
+        }
+
         [Fact, Trait(Traits.Feature, Traits.Features.Classification)]
         public async Task TestUnmanagedConstraint_LocalFunction_Keyword()
         {

src/EditorFeatures/TestUtilities/Classification/FormattedClassification.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System;
+using System.Linq;
 
 namespace Microsoft.CodeAnalysis.Editor.UnitTests.Classification
 {
@@ -35,6 +36,14 @@ public override int GetHashCode()
 
         public override string ToString()
         {
+            if (ClassificationName.StartsWith("regex"))
+            {
+                var remainder = ClassificationName.Substring("regex - ".Length);
+                var parts = remainder.Split(' ');
+                var type = string.Join("", parts.Select(Capitalize));
+                return "Regex." + $"{type}(\"{Text}\")";
+            }
+
             switch (ClassificationName)
             {
                 case "punctuation":
@@ -68,8 +77,11 @@ public override string ToString()
                     goto default;
 
                 default:
-                    return $"{char.ToUpperInvariant(ClassificationName[0])}{ClassificationName.Substring(1)}(\"{Text}\")";
+                    return $"{Capitalize(ClassificationName)}(\"{Text}\")";
             }
         }
+
+        private static string Capitalize(string val)
+            => char.ToUpperInvariant(val[0]) + val.Substring(1);
     }
 }