I had this question in my end semester exam, unfortunately I couldn't solve it and tried it for several days and no luck. 

Condition- For every substring of length 4 in a string of length n, write a RE to force the rule- there must be exactly three 1's.

My solution kind of looked like 
(1+11+111+€)(0111)\*(0+€).

But this is obviously wrong, string 11011 is a valid solution too.

Update- my new solution is (1+11+111+€)(0111)\*(0+01+011+€). 

Update- the plus operator is actually 'OR'

Update- € is empty string

Update - the string length has no requirements. A string of length 5 will have 2 substrings of length 4, the first 4 chars and the last 4 chars