dodo512

To use a literal instance of a special character in a regular expression, precede it by two backslash (\) characters. The MySQL parser interprets one of the backslashes, and the regular expression library interprets the other.

$i = 1;

$text = preg_replace_callback(
    '/<h3\K/i', 
    function () use(&$i) {
        return ' id="'.$i++.'"';
    },
    $text
);

$str = preg_replace('#<img[^>]+src="[^"]*?([^/"]+)\.gif"[^>]*>#i', ':$1', $str);

$text = preg_replace('/<a\s[^<>]*href="[^"]*\K\(/', '1', $text);

$text = preg_replace_callback(
    '/<a\s[^<>]*href="\K[^"]+/', 
    function ($m) {
        return str_replace('(', '1', $m[0]);
    },
    $text
);

RewriteRule ^(.*)ö(.*) https://site.ru/$1o$2 [R=301,L]
RewriteRule ^(.*)ü(.*) https://site.ru/$1u$2 [R=301,L]
RewriteRule ^(.*)õ(.*) https://site.ru/$1o$2 [R=301,L]
RewriteRule ^(.*)ä(.*) https://site.ru/$1a$2 [R=301,L]

$text = 'Просто текст 12345';
$matches = [];
$text = preg_replace_callback('~[0-9]~', function($match) use(&$matches) {
$matches[] = $match[0];
return '';
}, $text);

var_dump($text, $matches);

preg_match('/\x{fe0f}\x{20e3}/u', $str, $match);

var_dump($match);