Several Regex Modifiers in PHP

After the delimiter, you can write several modifiers. Example:

php<?php
  preg_replace('#[a-z]+#iU', '!', 'aaa bbb');
?>

Задание 1

Explain how the modifiers change the behavior of the regex in the following code:

php<?php
  preg_replace('#[a-z]+#Uu', '!', 'aaa bbb');
?>

Задание 2

Explain how the modifiers change the behavior of the regex in the following code:

php<?php
  preg_replace('#[a-z]+#Xx', '!', 'aaa bbb');
?>

Задание 3

Explain how the modifiers change the behavior of the regex in the following code:

php<?php
  preg_replace('#[a-z]+#ui', '!', 'aaa bbb');
?>