$re = '/(\d+)\n(\w+)\n([\d,.]+)/m';
$str = '1000
AAAAAAAAAA
100,000.00
2000
BBBBBBBBBB
200,000.00
3000
CCCCCCCCCC
300,000.00
4000
DDDDDDDDDD
400,000.00';
$subst = "$1,$2,$3";
$result = preg_replace($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