I have the following regular expression
$$(ΣΣ)^∗ = \{w\mid w \text{ is a string of even length}\}.$$
The problem is that I don't quiet agree with the description of the regular expression. It is indeed a string of even length, but strings like 1000 are not allowed. $x^*$ means all the strings in the form xxxxxx... , but there is no way you can get 1000 with something in the form $(x)^*$.