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