Kaktus Team

class LoginActivity extends Activity {
   ...
   private void startActivity2() {
      Intent intent = new Intent(this, Activity2.class);
      startActivity(intent);
      finish();
   }
   ...
}

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

public class Main {

    public static void main(String[] args) {
        String content = "В кабинет зашел [id000|Иван Иванов], а там уже сидит [id999|Петр Смирнов]";

        Pattern pattern = Pattern.compile("\\[id([0-9]+)\\|(.*?)\\]");
        Matcher m = pattern.matcher(content);

        StringBuffer sb = new StringBuffer();
        while (m.find()) {
            m.appendReplacement(sb, "<a href=\"vk.com/id$1\">$2</a>");
        }
        m.appendTail(sb);

        String result = sb.toString();
        System.out.println(result);
    }
}