$re = '/(?<!\w\.\w.)(?<![A-Z][a-z]\.)(?<=\.|\?)\s/m';
$str = 'Mr. Smith bought cheapsite.com for 1.5 million dollars, i.e. he paid a lot for it. Did he mind? Adam Jones Jr. thinks he didn\'t. In any case, this isn\'t true... Well, with a probability of .9 it isn\'t.';
$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