使用DOM解析器解析xml并添加限制

I am trying to use DOM parser for my xml.The code is as follows. It lists all the elements by tag type and its properties like price and color. However its hard time for me to find out how to add restrictions to the code such as "give me all the pens that are above " or "the name of pen whose color is black". Can someone help me out.I am trying to use DOM parser for my xml.The co