$re = '/(^<table.+.(?:[n])?(?:.+)?)(<tbody>)/m';
$str = '<table class="table table-bordered table-responsive" style="width: 100%;">
<tbody>';
$subst = "1<thead>";
$result = preg_replace($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