RegexUserName.java

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

/**
*   Give true is all condition matches, otherwise false.
*   @param  String 
*   @return boolean
*/ 
private boolean validateUserName(String username){
    Pattern pattern = Pattern.compile("^[a-z0-9_-]{3,15}$");
    Matcher matcher = pattern.matcher(username); 
    return matcher.matches();

}