Regular expression - Validate username in java

Regular expression of a user name:

 
^[a-z0-9_-]{4,15}$
Explanation: 
^: Start 
[a-z0-9_-]: allowed characters
{4,15}: the length must be between 4 and 15 characters at most

This regular expression checks whether a user name consists of lowercase only, of digit and symbol '_'. It is the most used filter in several websites.

import java.util.regex.Matcher; 
import java.util.regex.Pattern; 

public class regex_username{

 private Pattern pattern; 
 private Matcher matcher; 

 private static final String USERNAME_PATTERN = "^[a-z0-9_-]{4,15}$"; 

 public regex_username(){
 pattern = Pattern.compile(USERNAME_PATTERN); 
 }

 /**
 * Validate the username with a regular expression
 * @retourne true if the name is valid, otherwise false
 */
 public boolean validate(final String username){

 matcher = pattern.matcher(username); 
 return matcher.matches(); 

 }
}

Examples:
user_99 -> yes
User_99 -> non
aby -> non
superuser#99 -> non
snapper_84 -> yes
965dean -> yes