<?php
// Здесь ваша строка
$string = '=?koi8-r?B?3NTPINTFzcE=?=';
$pattern = '@(?<=\=\?)'
. '(?<encoding>[a-zA-Z0-9-]++)\?'
. 'B\?(?<base64_string>[a-zA-Z0-9=]++)'
. '(?=\?\=)@u';
if ($m['encoding'] && $m['base64_string']) {
}
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