The format looks simple and its matching semantics are genuinely surprising in several places.
Only one group applies
A crawler picks the single most specific matching User-agent group and ignores every other group entirely, including the wildcard one. Rules in User-agent: * do not apply to Googlebot if a User-agent: Googlebot group exists anywhere in the file — a very common misunderstanding that leaves intended rules silently inactive.
Longest match wins, not first
Where Allow and Disallow both match a URL, the rule with the longer path pattern wins regardless of order. Allow: /blog/public/ beats Disallow: /blog/ for a URL under the former.
Wildcards are limited
* matches any sequence and $ anchors the end. There is no character class, no alternation and no capture. Anything more expressive has to be handled elsewhere.
It is per-origin and per-port
https://example.com and http://example.com have separate robots.txt files, as does each subdomain and each non-standard port. A rule on one does not govern the others.
Try it: Robots.txt Tester on SeoWolf's Notepad.