Regex splitting resulting in an extra space, using \\s but not getting rid of it

I am trying to parse a string using regex, the string has opening parantheses and ending parantheses, between them small English letters are listed, separated by a comma. Each comma is followed by a space. Like this

import java.util.Arrays;
import java.util.Set;
import java.util.HashSet;
import java.io.*;

public class StringLetters {

	public static void main(String[] args) {
		try {
			BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
			String[] temp = br.readLine().split("\\s*(\\{|,|\\}|\\s)\\s*");

			for (int i = 0; i < temp.length; i++) {
				System.out.println("temp[" + i + "] ===>" + temp[i]);
			}

			Set<String> set = new HashSet<>();
			for (String a : temp) {
				set.add(a);
			}

			System.out.println(set.size());
		} catch (IOException ioe) {

		}
	}
}

And when I am passing {a, b, c} this as input

$ java StringLetters

  {a, b, c}

 temp[0] ===>
 temp[1] ===>a
 temp[2] ===>b
 temp[3] ===>c
 4

Other inputs can be {}, {s, h, a, n, o, n, o}, {h, e, a, l, h, t} ... etc

So when I am doing

   $ java StringLetters
    {}
    0

which is correct, for empty string I should be getting this 0.

The empty string at the 0th place in the temp array is not what i want, to get rid of that I am using \s inside the '(\{|,|\}|\s)' but that is not helping me here!!

If all you need are the single alphabets from the input, I would go with a different approach than splitting.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.text.ParseException;
import java.util.HashSet;
import java.util.Set;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Main {
	public static void main(String[] args) throws ParseException, IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		Pattern pattern = Pattern.compile("\\p{Alpha}");
		Matcher matcher = pattern.matcher(br.readLine());

		Set<String> set = new HashSet<>();

		while (matcher.find()) {
			System.out.println(matcher.group());
			set.add(matcher.group());
		}

		System.out.println(set.size());
	}
}

A sample run:

{a, b, c}
a
b
c
3

Another sample run:

{}
0

Note: \p{Alpha} stands for a single alphabet and can be replaced with [A-Za-Z]. Learn more about these patterns here. You would also like to check Java regex tutorial.

If you want to stick to your own way of doing it (i.e. splitting the string), you can do it as follows:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.text.ParseException;
import java.util.HashSet;
import java.util.Set;

public class Main {
	public static void main(String[] args) throws ParseException, IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		String[] temp = br.readLine().split("\\s*(\\{|,|\\}|\\s)\\s*");

		for (int i = 1; i < temp.length; i++) {// Start with index 1
			System.out.println("temp[" + i + "] ===>" + temp[i]);
		}

		Set<String> set = new HashSet<>();
		for (String a : temp) {
			if (!a.isBlank() && !a.isEmpty()) {// Check if the string is not empty or blank
				set.add(a);
			}
		}

		System.out.println(set.size());
	}
}

A sample run:

{a, b, c}
temp[1] ===>a
temp[2] ===>b
temp[3] ===>c
3

I've put comments in the code to make it easier for you to notice the changes. The reason why these changes are required is String#split returns an array with a minimum size of 1 e.g. System.out.println("Hello".split("$").length) will print 1.

The regex is seeing the { as the first delimiter, giving you an empty string. The simple way is to filter the array as you create the Set:

        Set<String> set = new HashSet<>(); 
        for(String a : temp){
            if (a != null && !a.isEmpty())
               set.add(a);
        }

Without calling the split the above task can be done as follows :-

import java.util.Set;
import java.util.HashSet;
import java.io.*;

public class StringLetters {
  
    public static void main(String[] args){
      
        try{
           
          BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
          String str = br.readLine();
          Set<Character> set = new HashSet<>();
          for(int i = 0; i < str.length(); i++){
            
              if(str.charAt(i) != '{' && str.charAt(i) != '}' && str.charAt(i) != ' ' && str.charAt(i) != ','){
                 
                 set.add(str.charAt(i));

              }
          }

          System.out.println(set.size());
        
        }catch(IOException ioe){
          ioe.printStackTrace();
        }
    }
}

The task itself is easy but I am interested in the regex, would like to know how this can be done using regex.