Define the matching pattern as a word boundry, a lowercase a, any number of immedietly trailing letters numbers, or underscores, followed by a word boundary
import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class MainClass {
public static void main(String args[]) throws Exception {
String candidate = "applying a pattern.";
String regex = "\\ba\\w*\\b";
Pattern p = Pattern.compile(regex);
Matcher m = p.matcher(candidate);
String val = null;
System.out.println("INPUT: " + candidate);
System.out.println("REGEX: " + regex + "\r\n");
while (m.find()) {
val = m.group();
System.out.println("MATCH: " + val);
}
if (val == null) {
System.out.println("NO MATCHES: ");
}
}
}