The function pinv calculates the pseudoinverse:
matlab link
Sometimes the matrix inv(A'*A) has a determinant that is close to 0, so it close to non invertible. In this case there could actually be a family of vectors that minimizes the least-squares distance.
Matlab points this out by warnings...something about the condition number too high. If matlab is fine with regress (no warnings), then regress is ok.
In financial application this happens sometimes when you have inputs that are highly correlated...I guess it's ok if you have a lot of inputs, like in OP-s case.