Closed Real Interval is Regular Closed

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

Let $\closedint a b$ be a closed interval of $\R$.

Then $\closedint a b$ is regular closed in $\struct {\R, \tau_d}$.

Proof
From Closed Real Interval is Closed in Real Number Line, $\closedint a b$ is closed in $\struct {\R, \tau_d}$.

From Interior of Closed Real Interval is Open Real Interval:
 * $\closedint a b^\circ = \openint a b$

where $\closedint a b^\circ$ denotes the interior of $\closedint a b$.

From Closure of Open Real Interval is Closed Real Interval:
 * $\openint a b^- = \closedint a b$

where $\openint a b^-$ denotes the closure of $\openint a b$.

Hence the result, by definition of regular closed.