private static String getPrefix(int offset, PsiFile file) {
               if (offset > 0) {
                 offset--;
               }
               final String text = file.getText();
               StringBuilder prefixBuilder = new StringBuilder();
               while (offset > 0 && (Character.isLetterOrDigit(text.charAt(offset)) || text.charAt(offset) == ':')) {
                 prefixBuilder.insert(0, text.charAt(offset));
                 if (text.charAt(offset) == ':') {
                   break;
                 }
                 offset--;
               }
               return prefixBuilder.toString();
             }
private static String getPrefix(int offset, PsiFile file)