Use regular expression on ReadyAPI event target

I am using ReadyAPI 2.2.0 and haven't upgraded yet.  I don't see a "event dialogue" so I'm not sure what you mean. But From the Events button at the upper right, next to the bell icon, click the button.

The Events window opens. Use "+" to add RequestFilter.filterRequest - I thought from your original message you already know this stuff. 

In your groovy script - assuming you use the GET processing like the above code I suggested,

def headers = request.requestHeaders

String theMethod = context.getProperty("httpMethod").toString()

String theGet = new String("GET")

def namelist = context.getPropertyNames()

// EXAMPLE: Set header to application/x-www-form-urlencoded in case GET

// clear out request content, set header Content-Type to application/x-www-form/urlencoded
if (theMethod.substring(0, 3) == theGet) 
{

     // Clear out the request content. Actually this probably fullfills the same
     // as "Remove Empty Content" in the REST Request Properties: tab
     context.setProperty("requestContent", '')
     request.requestContent = ''

	log.info "headers (right before adding  the urlencoded content type): " + headers

	log.info "headers size now: " + (request.requestHeaders.size())

	headers.put( "Content-Type", "application/x-www-form-urlencoded") // Replace values with those you need

	log.info "Added Content-Type: application/x-www-form-urlencoded"

        // finish setting the header...
	request.requestHeaders = headers

	log.info "headers size now: " + (request.requestHeaders.size())

	// display the header
	log.info "actual header is " + request.requestHeaders

}
else
{
//blah blah blah 2
}