Best way to search through long strings

Hi,

Im working on a project where I have to sift through the entire HTML code of a webpage, and extract everything that lies within a certain HTML tag.  Essentially, at least as I see it, I need to sort through the entire webpage html source and find all occurrences of a certain html tag.

What would be the best way to do this?  I want to be sure to do it efficiently as I will be searching through ~2500 lines of html for each time the script is called.

Thanks in advance.