$urlChar = self::EXT_LINK_URL_CLASS;
$text = preg_replace_callback(
'!(?: # Start cases
- (<a.*?</a>) | # m[1]: Skip link text
+ (<a[ \t\r\n>].*?</a>) | # m[1]: Skip link text
(<.*?>) | # m[2]: Skip stuff inside HTML elements' . "
(\\b(?:$prots)$urlChar+) | # m[3]: Free external links" . '
(?:RFC|PMID)\s+([0-9]+) | # m[4]: RFC or PMID, capture number