How to extract characters from the middle of a string?

[Marian Petrides]

If I have a string containing 25 characters, how do I extract the eight 
characters starting at position 5 in the string?

[In BASIC this would be left$(5,8)  or some such.]

This should be a simple thing to do but I can't figure it out and can't 
figure out which dictionary item applies.

Thanks.

M