Its just so terrible, it reads like a child trying to show off to you rather than trying to teach you anything.
Preg_ReplaceA pretty complicated topic as regular expressions are tricky to grasp for some people at first and are like a minute scripting language in themselves, so lets start with some easy text replacing.... or not in this case:
Example 1. Convert HTML to textOH SOMETHING NICE AND SIMPLE TO GET US STARTED THEN?
gets much worse
Example 2. Using backreferences followed by numeric literalsAbsolutely no idea what that means at all, something to do with dates judging by the code.
It should be
Example 1: Simple text replace
AS THATS WHAT 70% OF PEOPLE WILL BE USING IT FOR, you want something a bit smarter than standard replace but don't want to go the whole way and convert HTML just yet.