Regex with at least three even digits and odd length

Question

As the title said, I need to make a regex for a number with at least three even digits and odd length.
As an example, the number 248 should match.
Thank you!

Answer 1

Let $e$ denote any even digit and $o$ any odd digit. Let

$$E=\big((e+o)(e+o)\big)^*$$

and

$$O=(e+o)E\,.$$

Then the expression

$$\begin{align*} &E(eee+eeEe+eEee+eEeEe+eOeOe)E\\ &\qquad+O(eee+eeEe+eEee+eEeEe+eOeOe)O\\ &\qquad+E(eeOe+eOee+eOeEe+eEeOe)O\\ &\qquad+O(eeOe+eOee+eOeEe+eEeOe)E \end{align*}$$

should do the trick.