$re = '/^CN=(.*?)(?<!\\\\),.*/m';
$str = 'CN=(A) Gro up3,OU=OU-Path1,OU=OU-Path2,DC=domain,DC=local
CN=(A) Group1,OU=OU-Path1,OU=OU-Path2,DC=domain,DC=local
CN=(A) Gro\\,up4,OU=OU-Path1,OU=OU-Path2,DC=domain,DC=local
CN=(A) Group2,OU=OU-Path1,OU=OU-Path2,DC=domain,DC=local';
$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