here's a hint. you can use the function strstr, in a manner similar to this:
result_ptr = strstr(registrationString,":")
printf("the registration number is %s\n",(result_ptr + 1));
EDIT: I just noticed your other thread. People here (including me) are very cautious about people coming here wanting work done for them. I dont think you're really trying that, and even if you were i'm not doing your homework with this answer. my code snippet is not a suitable solution, but only illustrates the concept.
i could have pointed you to an entry for "strstr()" function at any C library reference site, but i think we want daniweb to be a little bit more than just a redirection service.
at any rate, the example i provided isnt much different than one found in any reference.