$re = '/<[^>]*>/';
$str = '</style></head><body dir="ltr" style="max-width:21cm;margin-top:1cm; margin-bottom:1cm; margin-left:1cm; margin-right:1cm; ">
<p class="P11">PRE-REGISTRATION</p>
<p class="P11">FIL_RI_CHOP</p>
<p class="P1"><span class="T3">We confirm the completion of the pre-registration CRF for the patient <span class="T1">1</span> enrolled in the study <span class="T1">FIL_RI_CHOP</span>.</span><a id="_GoBack"/><span class="T1"></span></p>
<p class="P1">
<span class="T3">Date: </span><span class="T1">2018-08-03<br/></span>
<span class="T3">Patient\'s Code: </span><span class="T1">1<br/></span>
<span class="T3">Center: </span><span class="T1">17_Terni_S.Maria<br/></span>
<span class="T3">User: </span><span class="T1">site_admin<br/></span></p><p class="P1">
<span class="T3">Uffici Studi FIL</span></p><p class="P1">
<span class="T3">Fondazione Italiana Linfomi ONLUS</span></p>';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($matches);
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