you can use find and perl. find will search the files
you want. perl will change their contents.
for instance, suppose you want to change all
paragraphs "<p>this is a paragraph" to
"<p>this is another paragraph" on all your
html files under your current directory:
$ find . -name \*.html |
xargs perl -i -p -e 'BEGIN{undef $/} s/<p>this is a paragraph/<p>this is another paragraph/sg'
now it's a question of tunning the regular expression
inside s///. most of the times you will want to ignore
spaces, then you do:
s/<p>\s*this is ...\s*/<p>\n new ... \n/sg
also, you might want to change something inside your
tags (some attribute, for example):
s/(<img[^>]*src="

x.gif(".*?>)/$1y.gif$2/sg
(the $2 part wasn't really necessary, but makes it
more understandable)
cya
--
pkiller