[eluser]adityamenon[/eluser]
Can you explain your problem in full detail? This is just an example. Are you having many strings with different letter prefixes, or is it SN all the time?
Codeigniter doesn't have a native function for this. In this particular case, I would simply remove SN using str_replace()
Code:
$input = 'SN1001770';
$output = str_replace('SN', $input);
echo 'SN '.$output;
