$re = '/(?<=[.])(?=[^\s])/m';
$str = 'The two platinum derivatives have been previously combined as they are not totally cross-resistant and as they share no overlapping toxicities.Patients with AOC, after the initial cytoreductive surgery were randomized to either 6 courses of paclitaxel at 175 mg/m2 as 3 h infusion plus Carboplatin at 7 AUC (Arm A) or Paclitaxel at the same dose plus Carboplatin again at 7 AUC for cycles 1,3,5, while for cycles 2,4,6 Cisplatin at 75 mg/m2 substituted for Carboplatin (Arm B).247 patients are analyzed.';
$subst = " ";
$result = preg_replace($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