There are numerous ways to do it.
(1) Depending on the format of your source, you can use Search and Replace with text editors like Notepad++ or others like in this video:
There are more guides like this on Youtube.
(2) Then there's a tool called Email Extractor Lite:
http://clients.surf7.net/downloads.php?action=displaycat&catid=2
Hosted version can be found for instance here:
https://eel.surf7.net.my/
Too big texts will break the browser though.
(3) Another tool to try:
http://free.groupfetch.com/products/egrab/index.php