org-plain-link-re

org-plain-link-re is a variable defined in `org.el'.
Its value is

"\\<\\(doi\\|elisp\\|f\\(?:ile\\(?:\\+\\(?:\\(?:emac\\|sy\\)s\\)\\)?\\|tp\\)\\|https?\\|m\\(?:ailto\\|essage\\)\\|news\\|shell\\):\\([^ 	\n()<>]+\\(?:([[:word:]0-9_]+)\\|\\([^[:punct:] 	\n]\\|/\\)\\)\\)"

Documentation:
Matches plain link, without spaces.