If iterations are ok for you, then try the following..

if(/<csp c="(\d+)">/)  {
$number= 1;
$iteration = $1;
$string .= '<colspec colnum="'. $number . '" colname="col' .  $number+
++ . '"/>' while ( $iteration-- );
}
print $string;
[download]

code is, first check whether format given is right, if it is then use some temporary variables, and do the string concatenation with the help of it.