Several Regex Modifiers in PHP

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

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

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

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

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

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

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

<?php
	preg_replace('#[а-яё]+#ui', '!', 'яяя ЙЙЙ ёёё');
?>