code: public static boolean isValidFieldName(String fieldName) {
          return fieldName != null &&
                 fieldName.length() > 0 &&
                 Utils.isLetter(fieldName.charAt(0)) &&
                 Utils.allAlphaNumUnderscore(fieldName);
      }
docs: Indicate if the given field name is valid. Currently, field names must begin with a
  letter and consist of all letters, digits, and underscores.
  
  @param fieldName Candidate field name.
  @return          True if the given name is valid for fields.