Generates a single random string matching an AST, without enumerating the full match set: makes one random choice at each node (which alternative, how many repeats, which codepoint) instead of walking every possibility.

Threads an explicit :rand state through generation rather than touching the process's global :rand state, so Xeger.random/2's :seed option is reproducible without side-affecting unrelated code sharing the same process.

An unbounded :rep node (*, +, {m,}) draws its repeat count uniformly from min..opts[:max_repeat] when given. Without :max_repeat there's no upper bound to draw uniformly from, so each repeat beyond the pattern's own minimum is instead a coin flip (p = 0.5) on whether to continue -- unbounded in principle, geometrically distributed (so almost surely short) in practice.