Help with regular expressions. Why is Cyrillic not accepted?

Question

PHP

if (preg_match_all("/(!пр)\s+(^[а-я])/i", $mess, $tok)) {

Here is the code, in the $mess variable, !pr k is stored, but for some reason it does not find it, such a problem is only with Russian letters.

Answer 1

Put a modifier u "/(!пр)\s+(^[а-я])/iu"
Video about modifiers

Answer 2

It is necessary to work with non-Latin letters using the standard mbstring extension
php.net/manual/ru/book.mbstring.php see the mb_ereg_... methods by itself when specifying the correct encoding.
Also try regular expression modifier u (when using utf8)
php.net/manual/en/reference.pcre.pattern.modifiers.php