admin and preg_match('/\A<[a-z][^>]*>/', $input)) {
return $input; # allow html input as is if privileged
}
$markup = [
'{<((?:\w+:|/).+?)>}' => '<$1>', # unescape link entities
'{<(?:https?://)?([^>\s|]+)>}' => '<$1 $1>', # unnamed link
'{<([^>\s|]+)[\s|]([^>]+)>}' => '$2', # hyperlink
"/\r\n?/" => "\n", # unix newlines
"/ +\n/" => "
", # trailing spaces for hard line break
"/\n/" => "\n", # newlines start paragraphs
'/_(? '$1', # italic
'/\*(? '$1', # bold
'/~(? '$1', # stricken
'/`(? '$1', # monospace
];
$html = preg_replace(array_keys($markup), array_values($markup), htmlspecialchars($input));
return "
$html
";
}