#!/bin/bash
url="https://e...content-available-to-author-only...e.com/repository/local/foo-bar/2.15290.0/foo_bar-2.15290.0.tar.gz"
awk 'match($0,/2\.[0-9]+\.0/){print substr($0,RSTART+2,RLENGTH-4)}' <<< "$url"
IyEvYmluL2Jhc2gKdXJsPSJodHRwczovL2UuLi5jb250ZW50LWF2YWlsYWJsZS10by1hdXRob3Itb25seS4uLmUuY29tL3JlcG9zaXRvcnkvbG9jYWwvZm9vLWJhci8yLjE1MjkwLjAvZm9vX2Jhci0yLjE1MjkwLjAudGFyLmd6Igphd2sgJ21hdGNoKCQwLC8yXC5bMC05XStcLjAvKXtwcmludCBzdWJzdHIoJDAsUlNUQVJUKzIsUkxFTkdUSC00KX0nIDw8PCAiJHVybCI=