String line = "John,Smith,123 Main St,Lake City,AB,37221,john.smith@gmail.com,";
    Pattern ptn = Pattern.compile("(^[A-Za-z0-9.'_%+-]+@[A-Za-z0-9.-]+.[A-Za-z]{2,}$)",Pattern.CASE_INSENSITIVE);
    Matcher mtch = ptn.matcher(line);
    String newLine =  mtch.replaceFirst("abc@gmail.com");
    assertEquals("John,Smith,123 Main St,Lake City,AB,37221,abc@gmail.com,", newLine);

s.replaceFirst("[A-Za-z0-9.'_%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}", "abc@gmail.com")
                ^                                ^^             ^

s.replaceFirst("[\w.'%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}", "abc@gmail.com")