正規表現の uフラグ

regex.php

<?php
$subjects = [
    'ｚｅｎｋａｋｕ',
    'hankaku',
    'あいうえお'
];

foreach ($subjects as $subject) {
    preg_match('/^[ａ-ｚ]+$/', $subject, $match);
    var_dump($match);
}

echo '--------', PHP_EOL;

foreach ($subjects as $subject) {
    // `/u` をつける
    preg_match('/^[ａ-ｚ]+$/u', $subject, $match);
    var_dump($match);
}

% php regex.php                                                                                                  (:|✔ )
array(1) {
  [0]=>
  string(21) "ｚｅｎｋａｋｕ"
}
array(0) {
}
array(1) {
  [0]=>
  string(15) "あいうえお"
}
--------
array(1) {
  [0]=>
  string(21) "ｚｅｎｋａｋｕ"
}
array(0) {
}
array(0) {
}