function replaceWord (replaceMap, keepMap, rules) {
    return function (word) {
      // Get the correct token and case restoration functions.
      var token = word.toLowerCase();

      // Check against the keep object map.
      if (keepMap.hasOwnProperty(token)) {
        return restoreCase(word, token);
      }

      // Check against the replacement map for a direct word replacement.
      if (replaceMap.hasOwnProperty(token)) {
        return restoreCase(word, replaceMap[token]);
      }

      // Run all the rules against the word.
      return sanitizeWord(token, word, rules);
    };
  }
Replace a word with the updated word.

@param  {Object}   replaceMap
@param  {Object}   keepMap
@param  {Array}    rules
@return {Function}