Multiline Search and Replace in Atom editor

Try the regex [\s\S]*?, with your example <p>[\s\S]*?</p>

see also https://github.com/atom/find-and-replace/issues/303

The regular expression (.|\r?\n)*? is what you're looking for.

Used in the example above, <p>(.|\r?\n)*?</p> will select all three lines and you can then either replace or delete those lines.