Open Real Interval is Regular Open

Theorem
Let $\left({\R, \tau_d}\right)$ be the real number line under the usual (Euclidean) topology.

Let $\left({a, b}\right)$ be an open interval of $\R$.

Then $\left({a, b}\right)$ is regular open in $\left({\R, \tau_d}\right)$.

Proof
From Open Sets in Real Number Line, $\left({a, b}\right)$ is open in $\left({\R, \tau_d}\right)$.

From Closure of Open Real Interval is Closed Real Interval:
 * $\left({a, b}\right)^- = \left[{a, b}\right]$

where $\left({a, b}\right)^-$ denotes the closure of $\left({a, b}\right)$.

From Interior of Closed Real Interval is Open Real Interval:
 * $\left[{a, b}\right]^\circ = \left({a, b}\right)$

where $\left[{a, b}\right]^-$ denotes the interior of $\left[{a, b}\right]$.

Hence the result, by definition of regular open.