Not certain to be right, but :
When you create a regex, you have to cut/look inside the object you are searching for.
You have to know how it is made : you make a tool (split()) to detect the Pattern you like.
But when you use this tool, you want to make it usefull, so each time your pattern matches, you apply the rule pre-defined (replaceAll()).
How can you apply it if you do not know on what, this is not the same gesture/movement, so not the same class?
Forget you are a programmer and enrole you like an artisan, if now it is not obvious, find another point of view.
It is an language problem, each person have its meaning of words, often it is the same between them, sometime not, you have to guess what is behind the words wrotten which perturb your analysis.