/* package whatever; // don't place package name! */
 
import java.util.regex.*;
 
/* Name of the class has to be "Main" only if the class is public. */
class Main
{
	public static void main(String[] args) {
		// replace with "%" what was matched by group 1 
		// input: aaa123ccc
		// output: %123ccc
		System.out.println(replaceGroup("([a-z]+)([0-9]+)([a-z]+)", "aaa123ccc", 1, "%"));
 
		// replace with "!!!" what was matched the 4th time by the group 2
		// input: a1b2c3d4e5
		// output: a1b2c3d!!!e5
		System.out.println(replaceGroup("([a-z])(\\d)", "a1b2c3d4e5", 2, 4, "!!!"));
	}
 
	public static String replaceGroup(String regex, String source,
                                         int groupToReplace, String replacement) {
		return replaceGroup(regex, source, groupToReplace, 1, replacement);
	}
 
	public static String replaceGroup(String regex, String source,
                                      int groupToReplace, int groupOccurrence, String replacement) {
		Matcher m = Pattern.compile(regex).matcher(source);
		for (int i = 0; i < groupOccurrence; i++) {
			if (!m.find()) return source; // pattern not met, may also throw an exception here
		}
		return new StringBuilder(source)
                  .replace(m.start(groupToReplace), m.end(groupToReplace), replacement)
                      .toString();
	}
}

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