Can you please explain how this regex work in this code ?

See more:

Java

Java8

public class Math {
	public static void compare(String A, String B) {
		int a;
	    a= S.replaceAll("[^" + J + "]" , "").length();
	String b = S.replaceAll("[^"+ J +"]", "");
	    System.out.println(a is +a);
	    System.out.println(b is +b);
	    
	}

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		String A= "AHhB";
		String B="CKDADEGEGAh";
	Math.compare(J, S);
	}<pre lang="java">

}

What I have tried:

Output :a is 3
b is AAh
My Understanding : [^J] matches a characters which is not in String J.
But what is the use of “[^” and “]”