ExpressJet, www.expressjet.com is simply Continental's regional airline partner. It used to be wholly owned I believe, but not any more. They operate ERJ-135/145/XRs.
I have noticed that there's no telling which domestic route you might get a ERJ on. For example, Newark to Oklahoma city, 1335 miles (according to Continental), is on an ERJ. Newark to Freeport Bahamas is on an ERJ starting December. So even long routes can be on ERJs.