My favourite way of doing this is "splitting and popping":
var str = "test_23";
alert(str.split("_").pop());
// -> 23
var str2 = "adifferenttest_153";
alert(str2.split("_").pop());
// -> 153
split() splits a string into an array of strings using a specified separator string.
pop() removes the last element from an array and returns that element.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…