Oh My Girl!

<?php
$text = $_GET['text'];
$key = $_GET['key'];

$keyPattern = $key[0];
if (!ctype_alnum($key[0])) {
    $keyPattern = '\\' . $key[0];
}

$lastIndex = strlen($key) - 1;
for ($i = 1; $i < $lastIndex; $i++) {
    $char = $key[$i];
    if (ctype_digit($char)) {
        $keyPattern .= '\d*';
    } else if (ctype_lower($char)) {
        $keyPattern .= '[a-z]*';
    } else if (ctype_upper($char)) {
        $keyPattern .= '[A-Z]*';
    } else {
        $keyPattern .= '\\' . $char;
    }
}

$char = $key[$lastIndex];
if (ctype_alnum($char)) {
    $keyPattern .= $char;
} else {
    $keyPattern .= '\\' . $char;
}

$pattern = '/' . $keyPattern . '(.{2,6})' . $keyPattern . '/';

$matches = [];
preg_match_all($pattern, $text, $matches);

echo implode('', $matches[1]);