Compare 2 strings & determine the similarity between them in percentage

StringSimilarity.cs

public float SimilarityPercentage(string a, string b)
{
    var SpecialCharacters = new Regex("[^\\w\\d\\s]");

    a = SpecialCharacters.Replace(a, string.Empty);
    b = SpecialCharacters.Replace(b, string.Empty);

    var PairsInA = a.Split(' ').SelectMany(s => s.Zip(s.Substring(1), (fc, sc) => string.Concat(fc, sc)));
    var PairsInB = b.Split(' ').SelectMany(s => s.Zip(s.Substring(1), (fc, sc) => string.Concat(fc, sc)));

    return 2 * PairsInA.Intersect(PairsInB).Count() / PairsInA.Union(PairsInB).Count();
}