Variable: tramp-user-regexp
tramp-user-regexp is a variable defined in tramp.el.gz.
Value
"[^/:|[:blank:]]+"
Documentation
Regexp matching user names.
Source Code
;; Defined in /usr/src/emacs/lisp/net/tramp.el.gz
(defconst tramp-user-regexp (rx (+ (not (any "/:|" blank))))
"Regexp matching user names.")