src/Pure/General/completion.scala

     private def word: Parser[String] = word_regex
     private def word3: Parser[String] = "[a-zA-Z0-9_'.]{3,}".r
     private def underscores: Parser[String] = "_*".r
 
     def is_word(s: CharSequence): Boolean = word_regex.pattern.matcher(s).matches
-    def is_word_char(c: Char): Boolean = Symbol.is_ascii_letdig(c) || c == "."
+    def is_word_char(c: Char): Boolean = Symbol.is_ascii_letdig(c) || c == '.'
 
     def extend_word(text: CharSequence, offset: Text.Offset): Text.Offset =
     {
       val n = text.length
       var i = offset