/** @file list2103.cpp */
/** Listing 21-3. Sanitizing a String by Transforming It */
/** Test for non-letter.
 * @param ch the character to test
 * @return true if @p ch is not a character that makes up a word
 */
bool non_letter(char ch)
{
  return not isalnum(ch, std::locale());
}

/** Convert to lowercase.
 * Use a canonical form by converting to uppercase first,
 * and then to lowercase.
 * @param ch the character to test
 * @return the character converted to lowercase
 */
char lowercase(char ch)
{
  return std::tolower(ch, std::locale());
}

/** Sanitize a string by keeping only alphabetic characters.
 * @param str the original string
 * @return a santized copy of the string
 */
std::string sanitize(std::string str)
{
  // Remove all non-letters from the string, and then erase them.
  str.erase(std::remove_if(str.begin(), str.end(), non_letter),
            str.end());

  // Convert the remnants of the string to lowercase.
  std::transform(str.begin(), str.end(), str.begin(), lowercase);

  return str;
}