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 = "^\\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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