import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class Main {
public static void main(String[] args) {
CharSequence inputStr = "a b c a b c";
String patternStr = "a";
String replacementStr = "x";
Pattern pattern = Pattern.compile(patternStr);
// Replace all occurrences of pattern in input
Matcher matcher = pattern.matcher(inputStr);
String output = matcher.replaceAll(replacementStr);
System.out.println(output);
}
}