Sounds like a homework assignment to me, so I'll try to be vague.
If you are not allowed to use existing helper functions, you have to implement your own.
To solve this you need 3 functions:
1) find the length of a string, like strlen
2) find a string inside a string, like strstr
3) copy a part of one string to another, like strncpy
If you are stuck with the interface you described and have to work with that one char array given, you'll need to be extra careful not to stomp over input you still need.
I hope that helps.