What language can be created from a regular expression with nulls?
The regular expression $\emptyset$ matches nothing at all: not the empty string, not any non-empty string.

Any concatenation with $\emptyset$ also matches nothing: a string would only match $d\emptyset$ if it could be divided into some part that matches&nbsp;$d$ and some part that matches&nbsp;$\emptyset$, but no such part exists.  So $d\emptyset$ is the same thing as $\emptyset$: it matches nothing at all.

An alternation ("or") of something with&nbsp;$\emptyset$ has no effect: $c\epsilon+\emptyset$ means "anything that matches&nbsp;$c\epsilon$ or matches&nbsp;$\emptyset$" and that's just "anything that matches&nbsp;$c\epsilon$."

So $c\epsilon+d\emptyset$ is the same thing as&nbsp;$c\epsilon$ which, in turn, is the same thing as&nbsp;$c$.  The regular expression you quote is equivalent to the simple&nbsp;$c^*$.



Conceptually, $\emptyset$ plays a similar role to zero in addition and multiplication. Adding zero to anything doesn't change it, just as alternation of a regular expression with&nbsp;$\emptyset$ doesn't change it. Multiplying anything by zero makes it equal to zero, just as concatenation of any regular expression with&nbsp;$\emptyset$ turns it into&nbsp;$\emptyset$.

$\emptyset$ might seem rather pointless as a regular expression. It's needed because, without it, the empty language (the one with no strings at all in it, i.e., the set&nbsp;$\emptyset$) wouldn't be regular. To see this, it's not hard to prove by induction that every regular expression that doesn't contain the character&nbsp;$\emptyset$ must match at least one string. However, we want the empty language to be regular, because it's accepted by an automaton&nbsp;&ndash; in fact, by any automaton with no accepting states.