$re = '/<a.*(\[nggallery[^\]+]*\]).*?<\/a>/';
$str = '<a href="[nggallery id=xx]"><span class="shutterset"><img class="alignnone size-large wp-image-23067" title="Image Title" src="http://example.com/wp-content/uploads/2015/06/image-title.jpg" alt="" width="685" height="456" /></span></a>
<a href="[nggallery id=xxxxx]">Click here!</a>
<a title="title title" href="[nggallery id=xxx]" target="_blank">Title Link Title Link</a>';
$subst = "$1";
$result = preg_replace($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