使用regex删除相同类型的html嵌套标记的最终解决方案?

I've been days trying to find a solution WITH regex (before somebody says it: I know I should been using the PHP DOM Document library or something alike, but let's take this as a theoretical question), looking answers up and I finally came up with what I'll show near the end of this question.I've been days trying to find a solution WITH r