Escape quotes in double-quoted string literals

Lexer.Tests/MLexerShould.cs

@@ -226,6 +226,16 @@ namespace Lexer.Tests
             Assert.AreEqual("just a 'string'", tokens[0].PureToken.Value);
         }
 
+        [Test]
+        public void ParseDoubleQuotedStringLiteralWithEscapedQuotes()
+        {
+            var sut = CreateLexer("\"just a \"\"string\"\"\"");
+            var tokens = sut.ParseAll();
+            Assert.AreEqual(2, tokens.Count);
+            Assert.AreEqual(TokenKind.DoubleQuotedStringLiteral, tokens[0].Kind);
+            Assert.AreEqual("just a \"string\"", tokens[0].PureToken.Value);
+        }
+
         [Test]
         public void ParseNumberStartingWithDot()
         {

Lexer/MLexer.cs

@@ -406,14 +406,38 @@ namespace Lexer
         {
             Window.ConsumeChar();
             var n = 0;
-            while (Window.PeekChar(n) != '"')
+            var pieces = new List<string>();
+            while (true)
             {
+                if (Window.PeekChar(n) == '"')
+                {
+                    if (Window.PeekChar(n + 1) == '"')
+                    {
+                        var piece = Window.GetAndConsumeChars(n);
+                        pieces.Add(piece);
+                        Window.ConsumeChar();
+                        Window.ConsumeChar();
+                        pieces.Add("\"");
+                        n = -1;
+                    }
+                    else
+                    {
+                        break;
+                    }
+                }
+                if (IsEolOrEof(Window.PeekChar(n)))
+                {
+                    throw new ParsingException("Unfinished double-quoted string literal.");
+                }
+
                 n++;
             }
 
-            var literal = Window.GetAndConsumeChars(n);
+            var lastPiece = Window.GetAndConsumeChars(n);
+            pieces.Add(lastPiece);
+            var total = string.Join("", pieces);
             Window.ConsumeChar();
-            return PureTokenFactory.CreateDoubleQuotedStringLiteral(literal);
+            return PureTokenFactory.CreateDoubleQuotedStringLiteral(total);
         }
 
         private PureToken ContinueParsingUnquotedStringLiteral()

Lexer/PureTokenFactory.cs

@@ -88,9 +88,14 @@
             return new PureToken(TokenKind.StringLiteral, "'" + EscapeStringLiteral(s) + "'", s, Window.Position);
         }
         
+        private string EscapeDoubleQuotedStringLiteral(string s)
+        {
+            return s.Replace("\"", "\"\"");            
+        }
+
         public PureToken CreateDoubleQuotedStringLiteral(string s)
         {
-            return new PureToken(TokenKind.DoubleQuotedStringLiteral, "\"" + s + "\"", s, Window.Position);
+            return new PureToken(TokenKind.DoubleQuotedStringLiteral, "\"" + EscapeDoubleQuotedStringLiteral(s) + "\"", s, Window.Position);
         }
 
         public PureToken CreateUnquotedStringLiteral(string s)