How to retrieve specific characters within rows of database column?
Your question's not brilliantly clear, but if you want the 2nd, 3rd and 4th characters in the column city, in the table zip, then use SUBSTR:

SELECT SUBSTR(city,2,3) 
FROM student.zip;


REGEXP_INSTR returns the location of a match, not a string, so you don't want to use that. REGEXP_MATCH would be the one to use, but SUBSTR is simpler.