$re = '/(?<=[a-z])\.(?=\s*[A-Z])|[,:]/m';
$str = 'Power supply will be suspended in the following areas on 13-04-15 between 9.00 A.M. to 2.00 P.M. for maintenance work. NUNGAMBAKKAM AREA : Nungambakkam High road(Door no. 33 to 66), Ponnankipuram, Avenue road, New street. TI CYCLE AREA : Cholapuram Main road, Complete Thiruvenkata nagar, Stedford Hospital, Lenin nagar, Teachers colony, Sivananda nagar, Municipal Court and CTH road, Post office, Railway station road, School area, Ramapuram, South park st, Ambattur market, Thasildhar office';
$subst = "\n";
$result = preg_replace($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