Hmm.. from perlre:
Alternatives are tried from left to right, so the first alternative found for which the entire expression matches, is the one that is chosen. This means that alternatives are not necessarily greedy. For example: when matching `foo|foot' against "barefoot", only the "foo" part will match, as that is the first alternative tried, and it successfully matches the target string.
So that would take a string random with respect to length.
Update: I see. Can you explain that regex-feature?