用eclipse实现最最简单的登录

package denglu;

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

public class denglu {

public static void main(String[] args) {
	System.out.println("用户名：");
	//scanner用于读取输入
	Scanner scan = new Scanner(System.in);
	String userName = scan.nextLine();
	System.out.println("密码：");
	Scanner scan1 = new Scanner(System.in);
	String userPwd = scan1.nextLine();
	//——regex正则规则
	String USERNAME_REGEX = "^(([0-9])|([a-zA-Z])|[A-Za-z0-9]){3,12}$";
	Pattern USERNAME_PATTERN = Pattern.compile(USERNAME_REGEX);
	String USERPWD_REGEX ="^[A-Za-z](([0-9])|([a-zA-Z])|[A-Za-z0-9]){5,17}$";
	Pattern USERPWD_PATTERN = Pattern.compile(USERPWD_REGEX);
	if (null == userName || "".equals(userName.trim())) {
		System.out.println("用户名不能为空");
	}
	Matcher userNameMatcher = USERNAME_PATTERN.matcher(userName);
	if (!userNameMatcher.matches()) {
		System.out.println("用户名格式错误");
	}
	if (null == userPwd || "".equals(userPwd.trim())) {
		System.out.println("密码不能为空");
	}
	Matcher userPwdMatcher = USERPWD_PATTERN.matcher(userPwd);
	if (!userPwdMatcher.matches()) {
		System.out.println("密码格式错误");
	}

}

}

正则表达式规则：