$re = '/.*?[a-z0-9][.?!](?= )/';
$str = 'Bali bombings: U.S. President George W. Bush amongst many others has condemned the perpetrators of the Bali car bombing of October 11. The death toll has now risen to at least 187.';
$subst = "\1";
$result = preg_replace($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