How to find in the text and highlight a certain letter at the beginning of words (php, regex)?

Question

PHP

For example me". Big or small - it doesn't matter, but they should remain the same.
Apple pit wash -> I blocko I ma wash

Answer 1

$re = '/(\w+?)(?:[^\w]|$)/siu';
$str = preg_replace_callback($re, function($m) {
    return "<span class='red'>". mb_substr($m[1], 0, 1) ."</span>". mb_substr($m[1], 1);
}, $str );

Answer 2

$str = ' Яблоко яма умываться ';

echo preg_replace('/\bЯ/iu', '<b>$0</b>', $str);

echo preg_replace('/(?<!\w)Я/iu', '<b>$0</b>', $str);

echo preg_replace('/Я/iu', '<b>$0</b>', $str);