$re = '/http:\/\/[a-zA-Z0-9-:\/.=?_]+,\s(http:\/\/[a-zA-Z0-9-:\/.=?_]+),\shttp:\/\/[a-zA-Z0-9-:\/.=?_]+/m';
$str = 'http://abc.com/main_img_a, http://abc.com/altimg_a_1, http://abc.com/altimg_a_2';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($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