String's replaceAll() method and escape characters

Question

The line

System.out.println("\\"); 

prints a single back-slash (\). And

System.out.println("\\\\"); 

prints double back-slashes (\\). Understood!

But why in the following code:

class ReplaceTest {     public static void main(String[] args)     {         String s = "hello.world";         s = s.replaceAll("\\.", "\\\\");         System.out.println(s);     } } 

is the output:

hello\world 

instead of

hello\\world 

After all, the replaceAll() method is replacing a dot (\\.) with (\\\\).

Can someone please explain this?

Answer 1

When replacing characters using regular expressions, you're allowed to use backreferences, such as \1 to replace a using a grouping within the match.

This, however, means that the backslash is a special character, so if you actually want to use a backslash it needs to be escaped.

Which means it needs to actually be escaped twice when using it in a Java string. (First for the string parser, then for the regex parser.)