Add Regex.Unescape Span overloads

Author: ViktorHofer

src/System.Text.RegularExpressions/ref/System.Text.RegularExpressions.cs

@@ -207,6 +207,7 @@ protected void InitializeReferences() { }
         void System.Runtime.Serialization.ISerializable.GetObjectData(System.Runtime.Serialization.SerializationInfo si, System.Runtime.Serialization.StreamingContext context) { }
         public override string ToString() { throw null; }
         public static string Unescape(string str) { throw null; }
+        public static int Unescape(ReadOnlySpan<char> str, Span<char> destination) { throw null; }
         protected bool UseOptionC() { throw null; }
         protected bool UseOptionR() { throw null; }
         protected internal static void ValidateMatchTimeout(System.TimeSpan matchTimeout) { }

src/System.Text.RegularExpressions/src/System/Text/RegularExpressions/Regex.cs

@@ -276,7 +276,17 @@ public static string Unescape(string str)
             if (str == null)
                 throw new ArgumentNullException(nameof(str));
 
-            return RegexParser.Unescape(str);
+            return RegexParser.Unescape(str.AsSpan(), Span<char>.Empty, false, out _);
+        }
+
+        /// <summary>
+        /// Unescapes any escaped characters in the input text.
+        /// </summary>
+        public static int Unescape(ReadOnlySpan<char> str, Span<char> destination)
+        {
+            RegexParser.Unescape(str, destination, true, out int charsWritten);
+
+            return charsWritten;
         }
 
         /// <summary>

src/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexParser.cs

@@ -159,36 +159,39 @@ public static string Escape(ReadOnlySpan<char> input, Span<char> destination, bo
         /*
          * Escapes all metacharacters (including (,),[,],{,},|,^,$,*,+,?,\, spaces and #)
          */
-        public static string Unescape(string input)
+        public static string Unescape(ReadOnlySpan<char> input, Span<char> destination, bool targetSpan, out int charsWritten)
         {
             for (int i = 0; i < input.Length; i++)
             {
                 if (input[i] == '\\')
                 {
-                    StringBuilder sb = StringBuilderCache.Acquire();
                     RegexParser p = new RegexParser(CultureInfo.InvariantCulture);
-                    int lastpos;
-                    p.SetPattern(input);
+                    p.SetPattern(input.ToString());
 
-                    sb.Append(input, 0, i);
+                    Span<char> charInitSpan = stackalloc char[256];
+                    var vsb = new ValueStringBuilder(charInitSpan);
+
+                    vsb.Append(input.Slice(0, i));
+                    int lastpos;
                     do
                     {
                         i++;
                         p.Textto(i);
                         if (i < input.Length)
-                            sb.Append(p.ScanCharEscape());
+                            vsb.Append(p.ScanCharEscape());
                         i = p.Textpos();
                         lastpos = i;
                         while (i < input.Length && input[i] != '\\')
                             i++;
-                        sb.Append(input, lastpos, i - lastpos);
+                        vsb.Append(input.Slice(lastpos, i - lastpos));
                     } while (i < input.Length);
 
-                    return StringBuilderCache.GetStringAndRelease(sb);
+                    return vsb.CopyOutput(destination, false, targetSpan, out charsWritten);
                 }
             }
 
-            return input;
+            // If nothing to escape, return the input.
+            return input.CopyInput(destination, targetSpan, out charsWritten);
         }
 
         /*

src/System.Text.RegularExpressions/tests/Regex.EscapeUnescape.Tests.cs

@@ -41,6 +41,11 @@ public void Escape_NullString_ThrowsArgumentNullException()
         public void Unescape(string str, string expected)
         {
             Assert.Equal(expected, Regex.Unescape(str));
+
+            // Use Escape(ReadOnlySpan, Span)
+            Span<char> output = stackalloc char[255];
+            int charsWritten = Regex.Unescape(str.AsSpan(), output);
+            SpanTestHelpers.VerifySpan(expected, charsWritten, output);
         }
 
         [Fact]