$re = '/(?P<number>\d+) (?P<word>\w+)/m';
$str = 'ABC 123 Book 456 ZZZ 789 KKK XYZ';
$subst = "\g<word> \g<number>";
$result = preg_replace($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