$re = '/Procedure:([\S\s]+)CLINICAL DIAGNOSIS/';
$str = 'SPECIMEN: Procedure: xxxx1 A) Location: yyyy2
Major zzz B) Location: something
text here C) more
CLINICAL DIAGNOSIS: xyz';
preg_match($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