SanitizeThat.java

public class SanitizeThat {
  public static String sanitizeIntegerToString(String input) {
    if (input == null)
      return "";

    String result = "";

    // Rule: Only digits except for "%" and "_"
    result = input.replaceAll("[^\\d\\%\\_]", "");
    result = stripInvalidWildcards(result);

    return result;
  }

  public static String sanitizeAlphaToString(String input) {
    if (input == null)
      return "";

    String result = "";

    // Rule: Only alpha characters except for "%" and "_"
    result = input.replaceAll("[^a-zA-Z\\%\\_]", "");
    result = stripInvalidWildcards(result);

    return result;
  }


  // This is the one I'm trying to refine.
  private static String stripInvalidWildcards(String input) {
    if (input == null)
      return "";

    String illegalPattern1 = "%%";
    String illegalPattern2 = "%_%";
    String result = input;

    while (result.contains(illegalPattern1) || result.contains(illegalPattern2)) {
      result = result.replaceAll(illegalPattern1, "%");
      result = result.replaceAll(illegalPattern2, "%");
    }

    if (result.equals("%") || result.equals("_"))
      return "";

    return result;
  }
}

Updated stripInvalidWildcards() with updated function.

Still needs improvement.