Fix GeneratedRegex fixer to preserve multiline verbatim string patterns

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

@@ -295,9 +295,9 @@ static RegexOptions GetRegexOptionsFromArgument(ImmutableArray<IArgumentOperatio
                         return SyntaxFactory.ParseExpression(optionsLiteral);
 
                     case UpgradeToGeneratedRegexAnalyzer.PatternArgumentName:
-                        if (argument.Value.ConstantValue.Value is string str && str.Contains('\\'))
+                        if (argument.Value.ConstantValue.Value is string str && ShouldUseVerbatimString(str))
                         {
-                            // Special handling for string patterns with escaped characters
+                            // Special handling for string patterns with escaped characters or newlines
                             string escapedVerbatimText = str.Replace("\"", "\"\"");
                             return SyntaxFactory.ParseExpression($"@\"{escapedVerbatimText}\"");
                         }
@@ -313,6 +313,13 @@ static RegexOptions GetRegexOptionsFromArgument(ImmutableArray<IArgumentOperatio
                 }
             }
 
+            static bool ShouldUseVerbatimString(string str)
+            {
+                // Use verbatim string syntax if the string contains backslashes or newlines
+                // to preserve readability, especially for patterns with RegexOptions.IgnorePatternWhitespace
+                return str.IndexOfAny(['\\', '\n', '\r']) >= 0;
+            }
+
             static string Literal(string stringifiedRegexOptions)
             {
                 if (int.TryParse(stringifiedRegexOptions, NumberStyles.Integer, CultureInfo.InvariantCulture, out int options))

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

@@ -1336,6 +1336,70 @@ static void Main(string[] args)
             await VerifyCS.VerifyCodeFixAsync(test, expectedFixedCode);
         }
 
+        [Fact]
+        public async Task MultilineVerbatimStringPreservedByFixer()
+        {
+            string test = """
+                using System.Text.RegularExpressions;
+
+                static class Class
+                {
+                    private static Regex r = [|new Regex(@"a
+                             b
+                             c", RegexOptions.IgnorePatternWhitespace)|];
+                }
+                """;
+
+            string expectedFixedCode = """
+                using System.Text.RegularExpressions;
+
+                static partial class Class
+                {
+                    private static Regex r = MyRegex();
+
+                    [GeneratedRegex(@"a
+                             b
+                             c", RegexOptions.IgnorePatternWhitespace)]
+                    private static partial Regex MyRegex();
+                }
+                """;
+
+            await VerifyCS.VerifyCodeFixAsync(test, expectedFixedCode);
+        }
+
+        [Fact]
+        public async Task MultilineStringConcatenationPreservedByFixer()
+        {
+            string test = """
+                using System.Text.RegularExpressions;
+
+                static class Class
+                {
+                    private const string foo = "bar";
+                    private static Regex r1 = [|new Regex(@"a        " + foo + @"
+                                                        b
+                                                        c", RegexOptions.IgnorePatternWhitespace)|];
+                }
+                """;
+
+            string expectedFixedCode = """
+                using System.Text.RegularExpressions;
+
+                static partial class Class
+                {
+                    private const string foo = "bar";
+                    private static Regex r1 = MyRegex();
+
+                    [GeneratedRegex(@"a        bar
+                                                        b
+                                                        c", RegexOptions.IgnorePatternWhitespace)]
+                    private static partial Regex MyRegex();
+                }
+                """;
+
+            await VerifyCS.VerifyCodeFixAsync(test, expectedFixedCode);
+        }
+
         [Fact]
         public async Task TestAsArgument()
         {