$re = '/^(\w+\s+\w+\(\w+\s+\*)\w+(\).*)$/';
$str = 'typedef size_t(__cdecl *pfn_fwrite_t)(const void *restrict buffer, size_t size, size_t count, FILE *restrict stream);';
$subst = "\1 \2";
$result = preg_replace($re, $subst, $str, 1);
echo "The result of the substitution is ".$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