>>106499635
C-h f rx
It has a detailed and well-organized docstring. Try doing simple things in the REPL, too. It'll help you get a feel for it.
ELISP> (setq p (rx "#" (group (= 2 hex)) (group (= 2 hex)) (group (= 2 hex))))
"#\\([[:xdigit:]]\\{2\\}\\)\\([[:xdigit:]]\\{2\\}\\)\\([[:xdigit:]]\\{2\\}\\)"
ELISP> (setq s "#655A7C")
"#655A7C"
ELISP> (string-match p s)
0
(#o0, #x0, ?\C-@)
ELISP> (match-string 1 s)
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