How to search HTML for key substrings

Question

I have a several blocks of html (full sources for websites) currently they are stored in a dataTable and I want to search each of them for image urls and key text then extract each of these pieces of data to there own string to then be filed in the DataTable (and eventually be put into the database). currently I have no code as this is a bit beyond me but my guest at how to do it is to use something to find groups of characters signifying the beginning and end of the substring I want and then using SubString(int, int) to extract it but this idea is all I have please help!