$re = '/(cla20.*)(\.h)/m';
$str = '#include "cla20ioh.h"
#include "cla20uth.h"
#include "cla20gch.h"
#include "cla20fnh.h"';
$subst = "\U\1\E\2";
$result = preg_replace($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