import java.util.regex.*;

public class BasicReplace
{
public static void main(String[] args)
{
CharSequence inputStr = "a b c a b c";
String patternStr = "a";
String replacementStr = "x";

// Compile regular expression
Pattern pattern = Pattern.compile(patternStr);

// Replace all occurrences of pattern in input
Matcher matcher = pattern.matcher(inputStr);
String output = matcher.replaceAll(replacementStr);
// x b c x b c
}
}