A regular polygon is one that has not only all sides the same measure, but all the interior angles the same measure as well. The 4 sides of a rhombus all have the same measure; but it is not necessary that the 4 interior angles have the same measure. So in general a rhombus cannot be said to be regular. However if the rhombus happens to be a square then all its interior angles are right angles and in that case, the square, which is a rhombus, will be regular:On the left is an ordinary rhombus, It is not a regular quadrilateral. On the right is a square, which is a rhombus, and a square is a regular quadrilateral. Edwin