Java String regex replace methods remove backslashes from replacement

Question

I just lost 1 hour to track what I consider not normal behaviour with replaceAll/replaceFirst in the String class.

If there is a backslash in the replacement String then they are removed when replacing. I then read that you can use Matcher.quoteReplacement(String) to create a proper replacement string,but my question is why? I can expect that the first argument should be escaped with Patter.quote(String) if you don't want the special meaning but I don't see a reason to change the replacement :(

Yes I will start using replace(CharSequence,CharSequence), just want to know why :)

Here is an example that clearly shows the "strange" behaviour:

public static void main(String[] args) {
    String out = "\\\\test\\\\";
    System.out.println(out);
    String result = "a".replaceAll("a", out);
    System.out.println(result);
}

note how second line is with only single backslashes as opposed to two like in the first line

Answer 1

Yes it is true that backslash needs to be doubly escaped as the first argument in String#replaceAll.

Reason:

It is due to the fact that your replacement String can contain back-references like $1, $2 etc even replacement text is also processed by underlying regex engine hence the need for double escaping same as the first argument, as you have also found out.