Privacy Browser Android

You are right in that this, apparently simple feature is quickly turned into a complicated mess when one starts thinking about all the possible URL modifications that the end-user might try to apply. So, I suggest thinking about the simple first steps to begin with.

Removing URL part: basically when that string is located in the URL, it is removed. It is up for the end user to be careful deciding what has to be removed.
Replacing URL part: whenever a certain string appears, it is replaced with another string.
Adding URL part: since we already covered "replacing", adding means basically at the end of the URL add some other string. It could also be done at the beginning of the URL, but maybe start with the end for now.

The same code used for removing Facebook tracking could be used to remove ANY part of the URL string and it shouldn't give any trouble (like I said, it's an advanced feature that leave up to the END USER the responsability of what he is deleting and wether or not the URL will load after).
All this would be a simple extra menu in the "Domain settings" in which you could have "add string" "remove string" and "replace string" options. For a start maybe have only ONE option available for a domain, at the same time.

Of course it's easier to say that do, but I hope this at least shows a clearer path of logic as to how this feature should work (for now at least, of course in the long run one could expect to use wildcards and such, but starting small and simple should help a lot).

This is a general implementation of the specific feature described at https://redmine.stoutner.com/issues/505. Likely one interface will be able to handle both.

There is also some discussion of this topic at https://redmine.stoutner.com/issues/844.

Allow user to use split, replace/replaceAll, slice, splice etc methods. Add this option to Domain list.

I am not sure I understand what you are requesting. However, I should make two points.

1. This feature request is about URL modification, not Domain Settings. If you are referring to Domain Settings, you should create a new feature request.
2. For performance reasons, I am unlikely to implement anything that would require processing regular expressions. Some of the words you are using make me think that perhaps you are requesting something that would fall into that category.

https://en.wikipedia.org/wiki/Regular_expression