Why String.replaceAll() in java requires 4 slashes “\\\\” in regex to actually replace “\”?

Question

I recently noticed that, String.replaceAll(regex,replacement) behaves very weirdly when it comes to the escape-character \"\\\"(slash)

For example consider there is

Answer 1

1) Let's say you want to replace a single \ using Java's replaceAll method:

   \
   ˪--- 1) the final backslash

2) Java's replaceAll method takes a regex as first argument. In a regex literal, \ has a special meaning, e.g. in \d which is a shortcut for [0-9] (any digit). The way to escape a metachar in a regex literal is to precede it with a \, which leads to:

 \ \
 | ˪--- 1) the final backslash
 |
 ˪----- 2) the backslash needed to escape 1) in a regex literal

3) In Java, there is no regex literal: you write a regex in a string literal (unlike JavaScript for example, where you can write /\d+/). But in a string literal, \ also has a special meaning, e.g. in \n (a new line) or \t (a tab). The way to escape a metachar in a string literal is to precede it with a \, which leads to:

\\\\
|||˪--- 1) the final backslash
||˪---- 3) the backslash needed to escape 1) in a string literal
|˪----- 2) the backslash needed to escape 1) in a regex literal
˪------ 3) the backslash needed to escape 2) in a string literal