We have received many questions about what the Error Status Code alert is for. This is a Website monitoring alert that is triggered based on http response code, as defined by the World Wide Web Consortium (W3C). The way its written uses regular expressions.
It is a very powerful alert that will alert you to any return status conditions of your website that are 400 or 500 errors. One of the most obvious status errors in the 400-599 range is the 404, or page not found.
For more on Regular Expressions or regexes, see: http://en.wikipedia.org/wiki/Regular_expression
For more on http status codes, see: http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html