Truncating Certain Characters

Is there any way in Oracle to only extract a certain number of charaters from a field.

For example, I have a student number field (STDNT_NUM), and some student numbers start with an S, such as

89187
S56789
S78900

I want a query that can either exclude the S, or that can extract the last 5 digits from the STDNT_NUM field. So, the data will look like this,

56789
78900

Is there a way to do this in Oracle?

Marcus