There is no issue number associated with these changes
This is my attempt to contribute to the project by improving the coding standard as a Joomla non-expert.
I don't honestly care how much of my stuff gets included into the core, but I just want to give my 2 cents and hope that decision makers will offer constructive feedback so that I can improve my contributions.

Summary of Changes

I have done a full-file review of just two randomly selected files. The intention is to make no behavioural changes. The changes are based on a range of benefits, such as:

1. improving code performance (replacing regex with non-regex techniques)
2. eliminating single-used variables and unnecessary overheads
3. replacing old array() syntax with modern [] and isset() calls with null coalescing
4. improving regex performance and reducing the data in the matches output / replacement arguments (several uses of \K)
5. removing unnecessary function calls (like empty() when a variable is assured to be declared/ then just checking for a falsey value)
6. making code more stable (2nd param on preg_quote() call)
7. remove anti-patterns (such as switch(true) block)
8. unset() can receive multiple arguments

Testing Instructions

none

Actual result BEFORE applying this Pull Request === Expected result AFTER applying this Pull Request

Documentation Changes Required

none