http://www.ScriptSocket.com/ Part 20 of the Perl Tutorial shows the functions length, substr, index and rindex.
https://wordpress.org/support/topic/converting-static-html-template-to-a-wp-website How to convert HTML Theme into WordPress Theme. how to create WordPress theme. you will learn Tutorial on how to Convert a static HTML template into a WordPress theme in-short converting HTML/CSS static page to a WordPress Theme.