This is a BNF-like description of the Uniform Resource Locator syntax, using the conventions of RFC822, except that "|" is used to designate alternatives, and brackets [] are used around optional or repeated elements. Briefly, literals are quoted with "", optional elements are enclosed in [brackets], and elements may be preceded with <n>* to designate n or more repetitions of the following element; n defaults to 0.
; The generic form of a URL is: genericurl = scheme ":" schemepart ; Specific predefined schemes are defined here; new schemes ; may be registered with IANA url = httpurl | ftpurl | newsurl | nntpurl | telneturl | gopherurl | waisurl | mailtourl | fileurl | prosperourl | otherurl ; new schemes follow the general syntax otherurl = genericurl ; the scheme is in lower case; interpreters should use case-ignore scheme = 1*[ lowalpha | digit | "+" | "-" | "." ] schemepart = *xchar | ip-schemepart ; URL schemeparts for ip based protocols: ip-schemepart = "//" login [ "/" urlpath ] login = [ user [ ":" password ] "@" ] hostport hostport = host [ ":" port ] host = hostname | hostnumber hostname = *[ domainlabel "." ] toplabel domainlabel = alphadigit | alphadigit *[ alphadigit | "-" ] alphadigit toplabel = alpha | alpha *[ alphadigit | "-" ] alphadigit alphadigit = alpha | digit hostnumber = digits "." digits "." digits "." digits port = digits user = *[ uchar | ";" | "?" | "&" | "=" ] password = *[ uchar | ";" | "?" | "&" | "=" ] urlpath = *xchar ; depends on protocol see section 3.1 ; The predefined schemes: ; FTP (see also RFC959) ftpurl = "ftp://" login [ "/" fpath [ ";type=" ftptype ]] fpath = fsegment *[ "/" fsegment ] fsegment = *[ uchar | "?" | ":" | "@" | "&" | "=" ] ftptype = "A" | "I" | "D" | "a" | "i" | "d" ; FILE fileurl = "file://" [ host | "localhost" ] "/" fpath ; HTTP httpurl = "http://" hostport [ "/" hpath [ "?" search ]] hpath = hsegment *[ "/" hsegment ] hsegment = *[ uchar | ";" | ":" | "@" | "&" | "=" ] search = *[ uchar | ";" | ":" | "@" | "&" | "=" ] ; GOPHER (see also RFC1436) gopherurl = "gopher://" hostport [ / [ gtype [ selector [ "%09" search [ "%09" gopher+_string ] ] ] ] ] gtype = xchar selector = *xchar gopher+_string = *xchar ; MAILTO (see also RFC822) mailtourl = "mailto:" encoded822addr encoded822addr = 1*xchar ; further defined in RFC822 ; NEWS (see also RFC1036) newsurl = "news:" grouppart grouppart = "*" | group | article group = alpha *[ alpha | digit | "-" | "." | "+" | "_" ] article = 1*[ uchar | ";" | "/" | "?" | ":" | "&" | "=" ] "@" host ; NNTP (see also RFC977) nntpurl = "nntp://" hostport "/" group [ "/" digits ] ; TELNET telneturl = "telnet://" login [ "/" ] ; WAIS (see also RFC1625) waisurl = waisdatabase | waisindex | waisdoc waisdatabase = "wais://" hostport "/" database waisindex = "wais://" hostport "/" database "?" search waisdoc = "wais://" hostport "/" database "/" wtype "/" wpath database = *uchar wtype = *uchar wpath = *uchar ; PROSPERO prosperourl = "prospero://" hostport "/" ppath *[ fieldspec ] ppath = psegment *[ "/" psegment ] psegment = *[ uchar | "?" | ":" | "@" | "&" | "=" ] fieldspec = ";" fieldname "=" fieldvalue fieldname = *[ uchar | "?" | ":" | "@" | "&" ] fieldvalue = *[ uchar | "?" | ":" | "@" | "&" ] ; Miscellaneous definitions lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z" hialpha = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" | "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" | "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z" alpha = lowalpha | hialpha digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" safe = "$" | "-" | "_" | "." | "+" extra = "!" | "*" | "'" | "(" | ")" | "," national = "{" | "}" | "|" | "\" | "^" | "~" | "[" | "]" | "`" punctuation = "<" | ">" | "#" | "%" | <"> reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" hex = digit | "A" | "B" | "C" | "D" | "E" | "F" | "a" | "b" | "c" | "d" | "e" | "f" escape = "%" hex hex unreserved = alpha | digit | safe | extra uchar = unreserved | escape xchar = unreserved | reserved | escape digits = 1*digit