$re = '/^((?:\£(?:[1-9]\d{0,2}(?:,\d{3})*|[0-9]+)?(?:\.\d{1,2})?|[^\n.])+)\.(.*)/m';
$str = 'This string should not split as the only periods that appear are here £19.99 and also here.
This string should split right here. As the period is not part of a price or at the end of the string.
There is a price in this string £19.99, but it should only split at this point. As I want it to ignore periods in a price. ';
$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