Fix verbatim escaping in regex code generator (#115447)

Author: PranavSenthilnathan

src/libraries/System.Text.RegularExpressions/gen/UpgradeToGeneratedRegexCodeFixer.cs

@@ -298,7 +298,8 @@ static RegexOptions GetRegexOptionsFromArgument(ImmutableArray<IArgumentOperatio
                 }
                 else if (argument.Value.ConstantValue.Value is string str && str.Contains('\\'))
                 {
-                    return SyntaxFactory.ParseExpression($"@\"{str}\"");
+                    string escapedVerbatimText = str.Replace("\"", "\"\"");
+                    return SyntaxFactory.ParseExpression($"@\"{escapedVerbatimText}\"");
                 }
                 else
                 {

src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/UpgradeToGeneratedRegexAnalyzerTests.cs

@@ -905,6 +905,48 @@ public static string CollapseWhitespace(this string text) =>
             await VerifyCS.VerifyCodeFixAsync(test, expectedFixedCode);
         }
 
+        [Fact]
+        public async Task CodeFixForConstantPatternExpressionWithQuote()
+        {
+            // From https://github.com/dotnet/runtime/issues/104371
+            // When constant expression patterns need to be escaped, we generate
+            // a verbatim string literal. However, we still need to escape quotes.
+            string expression = """
+                "[" + @"\/:<>|" + "\"]"
+                """;
+
+            string test = $@"using System.Text;
+using System.Text.RegularExpressions;
+
+public class Program
+{{
+    public static void Main(string[] args)
+    {{
+        var isMatch = [|Regex.IsMatch("""", {expression})|];
+    }}
+}}";
+
+            string verbatimPattern = """
+                @"[\/:<>|""]"
+                """;
+
+            string fixedSource = @$"using System.Text;
+using System.Text.RegularExpressions;
+
+public partial class Program
+{{
+    public static void Main(string[] args)
+    {{
+        var isMatch = MyRegex().IsMatch("""");
+    }}
+
+    [GeneratedRegex({verbatimPattern})]
+    private static partial Regex MyRegex();
+}}";
+
+            await VerifyCS.VerifyCodeFixAsync(test, fixedSource);
+        }
+
         [Fact]
         public async Task RawStringLiteralSyntaxPreservedByFixer()
         {