import java.util.*;
import java.lang.*;
import java.io.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		String regex = "tree[0-9][a-z]?";
		String string = "tree1\n"
		 + "tree2\n"
		 + "tree1a\n"
		 + "tree1b\n"
		 + "tree2a\n"
		 + "tree2b\n"
		 + "atree1\n"
		 + "tree20\n"
		 + "tree2ab";
 
		Pattern patternSet = Pattern.compile("^tree[0-9][a-z]?$", Pattern.MULTILINE);
		Matcher matcher = patternSet.matcher(string);
 
		while (matcher.find()) {
		    System.out.println(matcher.group(0));
		}
	}
}

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