Fix GeneratedRegex fixer to preserve multiline verbatim string patterns

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

@@ -382,9 +382,9 @@ private static RegexOptions GetRegexOptionsFromArgument(ImmutableArray<IArgument
                         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}\"");
                         }
@@ -401,6 +401,13 @@ private static RegexOptions GetRegexOptionsFromArgument(ImmutableArray<IArgument
             return null;
         }
 
+        private 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;
+        }
+
         private 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

@@ -1339,6 +1339,67 @@ 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
+                {
+                    [GeneratedRegex(@"a
+                             b
+                             c", RegexOptions.IgnorePatternWhitespace)]
+                    private static partial Regex r { get; }
+                }
+                """;
+
+            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";
+
+                    [GeneratedRegex(@"a        bar
+                                                        b
+                                                        c", RegexOptions.IgnorePatternWhitespace)]
+                    private static partial Regex r1 { get; }
+                }
+                """;
+
+            await VerifyCS.VerifyCodeFixAsync(test, expectedFixedCode);
+        }
+
         [Fact]
         public async Task TestAsArgument()
         {