过滤其它字符只保留IP地址

/// <summary>
/// 将传入字符串过滤掉汉字只保留IP地址
/// </summary>
/// <param name="strCharacters"></param>
/// <returns></returns>
public string FiltrateCharacters(string strCharacters)
{
    string strReturnString = string.Empty;
    char[] chr = strCharacters.ToCharArray();
    Regex RegDecimal = new Regex("^[0-9.]+$");

    for (int i = 0; i < chr.Length; i++)
    {
        string strSingel = chr[i].ToString();
        if (RegDecimal.IsMatch(strSingel))
        {
            strReturnString += strSingel;
        }
    }

    return strReturnString;
}