From fe478c12bf424842b154987e1ee42ebff8da4857 Mon Sep 17 00:00:00 2001
From: Corentin GS <contact@corentings.dev>
Date: Mon, 1 Jun 2026 21:28:56 +0200
Subject: [PATCH] Fix escaped quote handling in PGN tag values

---
 game.go         | 16 +++++++++++++++-
 lexer.go        | 15 ++++++++++++---
 scanner_test.go | 34 ++++++++++++++++++++++++++++++++++
 3 files changed, 61 insertions(+), 4 deletions(-)

diff --git a/game.go b/game.go
index d1e4dc890..32c7eba8a 100644
--- a/game.go
+++ b/game.go
@@ -377,6 +377,20 @@ func (g *Game) FEN() string {
 	return g.pos.String()
 }
 
+// escapeTagValue escapes backslash and double-quote characters so that the
+// resulting string is safe to embed inside a PGN tag value.
+func escapeTagValue(v string) string {
+	var sb strings.Builder
+	for i := 0; i < len(v); i++ {
+		c := v[i]
+		if c == '\\' || c == '"' {
+			sb.WriteByte('\\')
+		}
+		sb.WriteByte(c)
+	}
+	return sb.String()
+}
+
 // String implements the fmt.Stringer interface and returns
 // the game's PGN.
 func (g *Game) String() string {
@@ -397,7 +411,7 @@ func (g *Game) String() string {
 
 	// Write tag pairs.
 	for _, tagPair := range tagPairList {
-		sb.WriteString(fmt.Sprintf("[%s \"%s\"]\n", tagPair.Key, tagPair.Value))
+		sb.WriteString(fmt.Sprintf("[%s \"%s\"]\n", tagPair.Key, escapeTagValue(tagPair.Value)))
 	}
 
 	// Append empty line after tag pairs as per definition
diff --git a/lexer.go b/lexer.go
index 06dae3e83..9eab1a099 100644
--- a/lexer.go
+++ b/lexer.go
@@ -409,13 +409,22 @@ func (l *Lexer) readChar() {
 
 func (l *Lexer) readTagValue() Token {
 	l.readChar() // skip opening quote
-	position := l.position
+	var value strings.Builder
 	for l.ch != '"' && l.ch != 0 {
+		if l.ch == '\\' {
+			next := l.peekChar()
+			if next == '"' || next == '\\' {
+				l.readChar() // skip backslash
+				value.WriteByte(l.ch)
+				l.readChar()
+				continue
+			}
+		}
+		value.WriteByte(l.ch)
 		l.readChar()
 	}
-	value := l.input[position:l.position]
 	l.readChar() // skip closing quote
-	return Token{Type: TagValue, Value: value}
+	return Token{Type: TagValue, Value: value.String()}
 }
 
 func (l *Lexer) readTagKey() Token {
diff --git a/scanner_test.go b/scanner_test.go
index c88160918..48d424b0a 100644
--- a/scanner_test.go
+++ b/scanner_test.go
@@ -328,3 +328,37 @@ func TestScannerMultiFromPosNoExpand(t *testing.T) {
 	scanner := NewScanner(reader)
 	validateExpand(t, scanner, expectedLastLines, expectedFinalPos)
 }
+
+func TestEscapedQuoteInTagValue(t *testing.T) {
+	const escapedQuoteGame = `[Event "Internet Section 08A g/8'+2\""]
+[Site "Dos Hermanas"]
+[Date "2004.03.08"]
+[Round "6"]
+[White "Di Berardino, Diego Rafael"]
+[Black "Mar, Fernando"]
+[Result "1/2-1/2"]
+[ECO "B96"]
+
+1. e4 c5 2. Nf3 1/2-1/2
+`
+
+	scanner := NewScanner(strings.NewReader(escapedQuoteGame))
+	if !scanner.HasNext() {
+		t.Fatal("scanner should report one game")
+	}
+	game, err := scanner.ParseNext()
+	if err != nil {
+		t.Fatalf("BUG: parser rejects spec-legal escaped quote in tag value: %v", err)
+	}
+
+	// The in-memory value should contain a literal double quote.
+	if game.GetTagPair("Event") != `Internet Section 08A g/8'+2"` {
+		t.Fatalf("expected unescaped quote in tag value, got %q", game.GetTagPair("Event"))
+	}
+
+	// Round-trip: String() must emit the escaped form again.
+	pgn := game.String()
+	if !strings.Contains(pgn, `[Event "Internet Section 08A g/8'+2\""]`) {
+		t.Fatalf("round-trip PGN does not contain escaped quote: %s", pgn)
+	}
+}