Extension - Strip a string

One day I wrote this convienient extension that removes letters, digits and/or symbols - as specified - from a string:

/// <summary>Removes characters of specified types.</summary>
/// <param name="source">The string to remove from.</param>
/// <param name="removeTypes">A string containing 'L' to remove letters, 'D' to remove digits and 'S' to remove symbols.</param>
/// <returns>A string containing the remaining characters.</returns>
public static string Strip(this string source, string removeTypes) {
  bool letters = !string.IsNullOrEmpty(removeTypes) && removeTypes.Contains('L');
  bool digits = !string.IsNullOrEmpty(removeTypes) && removeTypes.Contains('D');
  bool symbols = !string.IsNullOrEmpty(removeTypes) && removeTypes.Contains('S');

  StringBuilder result = new StringBuilder();
  if (!string.IsNullOrEmpty(source)) {
    foreach (char c in source) {
      if (char.IsLetter(c)) {
        if (!letters) result.Append(c);
      }
      else if (char.IsDigit(c)) {
        if (!digits) result.Append(c);
      }
      else {
        if (!symbols) result.Append(c);
      }
    }
  }
  return result.ToString();
}

Note that the line removeTypes.Contains(char) relies on System.Linq.