dotv2

import java.util.regex.Pattern;
import java.util.regex.Matcher;

class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        Pattern pattern = Pattern.compile("\\b(\\d+)((?:\\d{3})+)\\b");
        Matcher matcher = pattern.matcher("sanyi1234 12345678910 másiksanyi12 142857142");
        System.out.println( matcher.replaceAll("$1.$2") );
    }
}