Why does replaceAll fail with “illegal group reference”?

Question

I am in need to replace

\\\\\\s+\\\\$\\\\$ to $$

I used

String s = \"  $$\";
s = s.replaceAll(\"\\\\s+\\\\$\\\\$\",\"$$\"         


        

Answer 1

$ has special meaning in the replacement string as well as in the regex, so you have to escape it there, too:

s=s.replaceAll("\\s+\\$\\$", "\\$\\$");