import java.util.*;
import java.lang.*;
import java.io.*;
import java.util.regex.*;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		String regex = "(?:\\G(?!^)|^(?=(?:[0-9a-fA-F]{2})*$))[0-9a-fA-F]{2}";
		String[] strings = {"aabb","_aabb","aa_bb", "aabb_"}; 
		Pattern pattern = Pattern.compile(regex);
		for (String s : strings) {
			System.out.println("Checking " + s);
			Matcher matcher = pattern.matcher(s);
			List<String> res = new ArrayList<>();
			while (matcher.find()) {
			    res.add(matcher.group(0));
			}
			if (res.size() > 0) {
				System.out.println(res);
			} else {
				System.out.println("No match!");
			}
		}
	}
}

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