Fix parsing of reserved chars

lexer/lexer_test.go

@@ -3133,6 +3133,14 @@ a: |invalid`,
 			name: "invalid document header option number",
 			src:  "a: >3\n  1",
 		},
+		{
+			name: "use reserved character @",
+			src:  "key: [@val]",
+		},
+		{
+			name: "use reserved character `",
+			src:  "key: [`val]",
+		},
 	}
 	for _, test := range tests {
 		t.Run(test.name, func(t *testing.T) {

parser/parser_test.go

@@ -1212,6 +1212,18 @@ a:
            ^
    6 |       d: e`,
 		},
+		{
+			"key: [@val]",
+			`
+[1:7] found invalid token
+>  1 | key: [@val]
+             ^
+`,
+		},
+		{
+			"key: [`val]",
+			"\n[1:7] found invalid token\n>  1 | key: [`val]\n             ^\n",
+		},
 	}
 	for _, test := range tests {
 		t.Run(test.source, func(t *testing.T) {

scanner/scanner.go

@@ -976,6 +976,19 @@ func (s *Scanner) scanAlias(ctx *Context) bool {
 	return true
 }
 
+func (s *Scanner) scanReservedChar(ctx *Context, c rune) error {
+	if ctx.existsBuffer() {
+		return nil
+	}
+
+	ctx.addBuf(c)
+	ctx.addOriginBuf(c)
+	err := ErrInvalidToken("%q is a reserved character", token.Invalid(string(ctx.obuf), s.pos()))
+	s.progressColumn(ctx, 1)
+	ctx.clear()
+	return err
+}
+
 func (s *Scanner) scan(ctx *Context) error {
 	for ctx.next() {
 		c := ctx.currentChar()
@@ -1103,6 +1116,10 @@ func (s *Scanner) scan(ctx *Context) error {
 			if s.scanWhiteSpace(ctx) {
 				continue
 			}
+		case '@', '`':
+			if err := s.scanReservedChar(ctx, c); err != nil {
+				return err
+			}
 		}
 		ctx.addBuf(c)
 		ctx.addOriginBuf(c)