CurrencyRegex.java

import java.util.*;
import java.util.regex.*;

public class CurrencyRegex {
	public static void main(String[] args) {
		for (String arg : args) {
			System.out.println(toCurrency(arg));
		}
	}

	public static String toCurrency(String str_num) {
		Pattern p = Pattern.compile("(?<!\\.\\d)(?<=\\d)(?=(?:\\d\\d\\d)+(?!\\d))");
		Matcher m = p.matcher(str_num);
		StringBuffer sb = new StringBuffer();
		while(m.find()) {
			m.appendReplacement(sb, ",");
		}
		m.appendTail(sb);
		return sb.toString();
	}
}