Can a regular expression be any string from the language described by it?
Your question is unclear. However &quot;a language (that has strings of infinite length)&quot; cannot exist. By definition a language only contains words of finite length.
Let $\Sigma$ be a (finite) alphabet and $\epsilon$ denote the empty word.
A language is a subset of $\Sigma^*$, where $\Sigma^*$ is defined as $\Sigma^* = \cup_{i=1}^{\infty} \Sigma^i$, $\Sigma^0 = \{ \epsilon \}$, and $\Sigma^i = \{xy : x \in \Sigma^{i-1}, y \in \Sigma\}$ for $i&gt;0$.
As you can see, a string of infinite length does not belong to any $\Sigma^i$ and therefore it belongs to neither $\Sigma^*$ nor to any of its subsets.