https://theriault.github.io/pcre-syntax/
This repo uses syntax diagrams to visually explain the syntax of Perl Compatible Regular Expressions.
PCRE is used by Apache, nginx, postfix, journalctl, Nmap, PHP, and others.
It is likely easier to learn PCRE experimenting with any of the above tools, Regex101, Regexr, RegexTester, etc.
This repo was an experiment — some of the diagrams get pretty gnarly.