var textToBeModified = "lolfoo123456789";
var replaceRule = new ReplaceRule
{
    RegexPattern = @"(?<=foo)\d{3}",
    ReplacementText = "bar"
};

var replaceResult = Regex.Replace(textToBeModified, replaceRule.RegexPattern, replaceRule.ReplacementText, RegexOptions.None);

Console.WriteLine(replaceResult); // lolfoobar45679