$re = '/(\s+)((?<=\s)[a-z]|^[a-z]).*?([A-Z]?(?<=[a-zA-Z0-9])[a-zA-Z0-9]+)/';
$str = 'do we have a Hashtag
';
$subst = "$2$3";
$result = preg_replace($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