Finds any value inside the HTML ... in order to replace with $...$

The string must be less than 21 characters, mus not have spaces, it can only contain lowercase letters, hyphens (-) or underscores (_)

Add dots BEHIND the y to make the selection go backwards (the y can be anything you want) and add dots BEFORE the \K to make the selection go forwards. The dot after the \K is to make the selection a whole letter.

First matches the children specified opened or self-closed tags and their contents. If not found, matches opening tag. If not found, matches closing tag.

Extract folder name and markers from a list of IMAP folders.

Used matching to http links within a page_source that are either jpeg or gif formats