import java.util.*;
import java.lang.*;
import java.io.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		final String regex = "(?U)^\\p{L}\\w{2,23}\\p{Alnum}$";
        final String string = "_notthis\n"
	 + "nothis_\n"
	 + "yes_123";
 
        final Pattern pattern = Pattern.compile(regex, Pattern.MULTILINE);
        final Matcher matcher = pattern.matcher(string);
 
        while (matcher.find()) {
            System.out.println(matcher.group(0));
        }
	}
}

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