Use literal string for regexp pattern

lexer.go

diff --git a/lexer.go b/lexer.go
index 3c7641a..bd23665 100644
--- a/lexer.go
+++ b/lexer.go
@@ -591,7 +591,7 @@
 }
 
 func init() {
-	dateRegexp = regexp.MustCompile("^\\d{1,4}-\\d{2}-\\d{2}T\\d{2}:\\d{2}:\\d{2}(\\.\\d{1,9})?(Z|[+-]\\d{2}:\\d{2})")
+	dateRegexp = regexp.MustCompile(`^\d{1,4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(\.\d{1,9})?(Z|[+-]\d{2}:\d{2})`)
 }
 
 // Entry point