"与えられた文字列中に含まれる単語の個数を単語ごとにカウントする"LINQ

WordCountByLinq.linq

<Query Kind="Expression" />

"I have a pen. I have a apple. oh!! Apple pen! I have a pen. I have a pineapple. oh!! Pineapple pen! Apple pen. Pineapplepen. Pen pineapple apple pen."
  .Split(' ')
  .Select(s => Regex.Replace(s, "[\\.!]", ""))
  .Select(s => s.ToLower())
  .ToLookup(w => w)
  .Select(g => new { Word = g.Key, Count = g.Count()})

// Word         | Count
// ------------ | ------
// i            | 4
// have         | 4
// a            | 4
// pen          | 7
// apple        | 4
// oh           | 2
// pineapple    | 3
// pineapplepen | 1