Parse long unicode

lexer.go

diff --git a/lexer.go b/lexer.go
index abb6551..e6c3566 100644
--- a/lexer.go
+++ b/lexer.go
@@ -395,6 +395,23 @@
 					return l.errorf("invalid unicode escape: \\u" + code)
 				}
 				growingString += string(rune(intcode))
+			case 'U':
+				l.pos++
+				code := ""
+				for i := 0; i < 8; i++ {
+					c := l.peek()
+					l.pos++
+					if !isHexDigit(c) {
+						return l.errorf("unfinished unicode escape")
+					}
+					code = code + string(c)
+				}
+				l.pos--
+				intcode, err := strconv.ParseInt(code, 16, 64)
+				if err != nil {
+					return l.errorf("invalid unicode escape: \\U" + code)
+				}
+				growingString += string(rune(intcode))
 			default:
 				return l.errorf("invalid escape sequence: \\" + string(l.peek()))
 			}

lexer_test.go

diff --git a/lexer_test.go b/lexer_test.go
index e9d0f14..683d7f7 100644
--- a/lexer_test.go
+++ b/lexer_test.go
@@ -458,6 +458,12 @@
 		token{Position{1, 8}, tokenString, "hello ♥"},
 		token{Position{1, 21}, tokenEOF, ""},
 	})
+	testFlow(t, `foo = "hello \U000003B4"`, []token{
+		token{Position{1, 1}, tokenKey, "foo"},
+		token{Position{1, 5}, tokenEqual, "="},
+		token{Position{1, 8}, tokenString, "hello δ"},
+		token{Position{1, 25}, tokenEOF, ""},
+	})
 }
 
 func TestLiteralString(t *testing.T) {