PHP: Strip everything except a match - php

Question

The following native function of my script does strip some content from my article such as bbcodes, html tags, and http or https websites, and I'd like to modify it so that it does strip everything except anything which matches "https://my.website.com/variablepath".
function _bbcode_strip($text)
{
static $patterns = array();
if ($this->tp_bbcode)
{
// use text inside [topicpreview] bbcode as the topic preview
if (preg_match('#\[(topicpreview[^\[\]]+)\].*\[/\1\]#Usi', $text, $matches))
{
$text = $matches[0];
}
}
$text = smiley_text($text, true); // display smileys as text :)
$text = ($this->tp_line_breaks ? str_replace("\n", '
', $text) : $text); // preserve line breaks
// Loop through text stripping inner most nested BBCodes until all have been removed
$regex = '#\[(' . $this->strip_bbcodes . ')[^\[\]]+\]((?:(?!\[\1[^\[\]]+\]).)+)\[\/\1[^\[\]]+\]#Usi';
while(preg_match($regex, $text))
{
$text = preg_replace($regex, '', $text);
}
if (empty($patterns))
{
$patterns = array(
'#<!-- [lmw] --><a class="postlink[^>]*>(.*<\/a[^>]*>)?<!-- [lmw] -->#Usi', // Magic URLs
'#<[^>]*>(.*<[^>]*>)?#Usi', // HTML code
'#\[/?[^\[\]]+\]#mi', // Strip all bbcode tags
'#(http|https|ftp|mailto)(:|\&\#58;)\/\/[^\s]+#i', // Strip remaining URLs
'#"#', // Possible quotes from older board conversions
'#[\s]+#' // Multiple spaces
);
}
return trim(preg_replace($patterns, ' ', $text));
}
I tried myself working on this part of the script which is the regex which decides what to strip:
'#<!-- [lmw] --><a class="postlink[^>]*>(.*<\/a[^>]*>)?<!-- [lmw] -->#Usi', // Magic URLs
'#<[^>]*>(.*<[^>]*>)?#Usi', // HTML code
'#\[/?[^\[\]]+\]#mi', // Strip all bbcode tags
'#(http|https|ftp|mailto)(:|\&\#58;)\/\/[^\s]+#i', // Strip remaining URLs
'#"#', // Possible quotes from older board conversions
'#[\s]+#' // Multiple spaces
but I didn't manage to do what I'm trying to, so I came here to ask for help. Thank you.