// using CSS spec for identifiers (id and class): a-z0-9, -, _. NOT . (which is OK in html spec, but not css)
String h = "<div><p id='a1-foo_bar'>One</p><p class='b2-qux_bif'>Two</p></div>";
Document doc = Jsoup.parse(h);
Element el1 = doc.getElementById("a1-foo_bar");
assertEquals("One", el1.text());
Element el2 = doc.getElementsByClass("b2-qux_bif").first();
assertEquals("Two", el2.text());
Element el3 = doc.select("#a1-foo_bar").first();
assertEquals("One", el3.text());