[[w:en:Formal_grammar|Formal grammar]]
[http://www.cl.cam.ac.uk/~mgk25/iso-ebnf.html EBNF help]

ISBN 1234567890
ISBN 123456789x
ISBN      1 2 3-4-5 6789 X
ISBN 978-9999999999

RFC 822
PMID 822

----------------------------------------------------

[
	["url", "[[w:en:Formal_grammar|Formal grammar]]"],
	["url", "[http://www.cl.cam.ac.uk/~mgk25/iso-ebnf.html EBNF help]"],

	["url", "ISBN 1234567890"],
	["url", "ISBN 123456789x"],
	["url", "ISBN      1 2 3-4-5 6789 X"],
	["url", "ISBN 978-9999999999"],

	["url", "RFC 822"],
	["url", "PMID 822"]
]

----------------------------------------------------

Checks for links, ISBN, RFC and PMID.