Hello, can someone knowledgable help me understand why this code does not work? It seems replaceFirst(regex, replacement) doesn't work the way I expect it to from https://www.w3schools.com/java/ref_string_replacefirst.asp.

a = a.toLowerCase();
b = b.toLowerCase();
        
if(a.length() != b.length()) { return false; }
        
for(int i = 0; i < b.length(); i++) {
	a.replaceFirst(String.valueOf(b.charAt(i)), "");
	// a.replaceFirst(b.substring(i, i+1), "");
}

return a.isEmpty();

(I wanted to try to not use Arrays and Hashmaps)

import java.util.Scanner;

public class Solution {

static boolean isAnagram(String a, String b) {
    a = a.replaceAll("\\s", "").toLowerCase();
    b=b.replaceAll("\\s", "").toLowerCase();

    char[] ch1 = a.toCharArray();
    char [] ch2 = b.toCharArray();

   java.util.Arrays.sort(ch1);
   java.util.Arrays.sort(ch2);

   String s1 = new String(ch1);
   String s2= new String (ch2);

   if( s1.equals(s2)){
    return true;
   }
   else {
    return false;
   }

}

I did like this Found it more simly to transform it into a char array sort it and compare:

import java.io.; import java.util.;

public class Solution {

public static void main(String[] args) {

    Scanner in = new Scanner(System.in);
    String a = in.nextLine().toLowerCase();
    String b = in.nextLine().toLowerCase();
    in.close();

    char[] arrayA = a.toCharArray();
    char[] arrayB = b.toCharArray();

    Arrays.sort(arrayA); 
    Arrays.sort(arrayB); 

    if(Arrays.equals(arrayA, arrayB)){
        System.out.println("Anagrams");
    } else {System.out.println("Not Anagrams");}



}

}

import java.io.; import java.util.;

public class Solution {

public static void main(String[] args) {

    Scanner scanner = new Scanner(System.in) ;

    String word1 = scanner.next();
    scanner.nextLine();
    String word2 = scanner.next();

    word1 = word1.toLowerCase();
    word2 = word2.toLowerCase();

    HashMap<Character, Integer> map1 = new HashMap<>();
    HashMap<Character, Integer> map2 = new HashMap<>();

    if (word1.length() == word2.length()) {
        for (int i=0 ; i < word1.length() ; i++) {

            Character letter1 = word1.charAt(i) ;
            Character letter2 = word2.charAt(i) ;

            if (map1.containsKey(word1.charAt(i))) {

                map1.put(letter1, map1.get(letter1)+1);
            } else {

                map1.put(letter1, 1) ;
            }


            if (map2.containsKey(word2.charAt(i))) {

                map2.put(letter2, map2.get(letter2)+1);
            } else {

                map2.put(letter2, 1) ;
            }

        }

        if (map1.equals(map2)) {
        System.out.println("Anagrams");
        } else {
            System.out.println("Not Anagrams") ;
        }
    } else {
        System.out.println("Not Anagrams") ;
    }



}

}

boolean isAnagram(String a, String b) {

    char [] a1 = a.toLowerCase().toCharArray();
    char [] a2 = b.toLowerCase().toCharArray();
    if (a1.length==a2.length){
        for (int i=0;i<a1.length;i++){
            if ((b.toLowerCase()).contains(a1[i]+"")){
            int ind = b.toLowerCase().indexOf(a1[i]);
            int count1 = 0;
            int count2 = 0;
            for (int j=0;j<a2.length;j++){
                if (a1[i]==a1[j]){
                    count1++;
                }
                if (a2[ind]==a2[j]){
                    count2++;
                }
            }
            if (count1!=count2){
                return false;
            }
            }
            else{
                return false;
            }
        }
        return true;
    }
    else{
        return false;
    }
}