$re = '/(?<!\S)#([0-9a-zA-Z]+)/i';
$str = '#ত্বকের উজ্জ্বলতা কী?
#কীভাবে ধরে রাখবেন উজ্জ্বলতা?
#PRP (Vampire Facial) #Chemical Peel #Oxyfacial #Hydrafacial #Glutathione Injection';
$subst = "<a href=\"/hashtag/$1\">#$1</a>";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php