editing a HTML file

[Tracubik]

Hi all,

I'would like to make a script that automatically change some text in a 
html file.

I need to make some changes in the text of <p> tags

My question is: there is a way to just "update/substitute" the text in 
the html <p> tags or do i have to make a new modified copy of the html file?

To be clear, i'ld like to make something like this:

open html file
for every <p> tags:
   if "foo" in text:
     change "foo" in "bar"
close html file

any sample would be really appreciated
I'm really a beginner as you can see

Thanks