/* package whatever; // don't place package name! */
 
import java.lang.*;
 
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
	{
		final String regex = "(?<=abc)aa";
        final Pattern pattern = Pattern.compile(regex);
 
        final Matcher matcher = pattern.matcher("abcaa");
 
        final String subMatch;
        if (matcher.find()) {
            subMatch = matcher.group();
        } else {
            subMatch = null;
        }
 
        System.out.println(subMatch);
	}
}