code: @Nullable
      public String extractUsername(String email) {
          int indexAt = email.indexOf(AT_SYMBOL);
          if (indexAt == -1) {
              return null;
          }
          return email.substring(0, indexAt);
      }
docs: Extracts the username part from the email
  
  @param email to parse
  @return the username String if found, an empty String otherwise