This is a BNF-like description of the Relative Uniform Resource Locator syntax, using the conventions of RFC 822 [5], except that "|" is used to designate alternatives. Briefly, literals are quoted with "", parentheses "(" and ")" are used to group elements, 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.
This BNF also describes the generic-RL syntax for valid base URLs. Note that this differs from the URL syntax defined in RFC 1738 [2] in that all schemes are required to use a single set of reserved characters and use them consistently within the major URL components.
URL = ( absoluteURL | relativeURL ) [ "#" fragment ] absoluteURL = generic-RL | ( scheme ":" *( uchar | reserved ) ) generic-RL = scheme ":" relativeURL relativeURL = net_path | abs_path | rel_path net_path = "//" net_loc [ abs_path ] abs_path = "/" rel_path rel_path = [ path ] [ ";" params ] [ "?" query ] path = fsegment *( "/" segment ) fsegment = 1*pchar segment = *pchar params = param *( ";" param ) param = *( pchar | "/" ) scheme = 1*( alpha | digit | "+" | "-" | "." ) net_loc = *( pchar | ";" | "?" ) query = *( uchar | reserved ) fragment = *( uchar | reserved ) pchar = uchar | ":" | "@" | "&" | "=" uchar = unreserved | escape unreserved = alpha | digit | safe | extra escape = "%" hex hex hex = digit | "A" | "B" | "C" | "D" | "E" | "F" | "a" | "b" | "c" | "d" | "e" | "f" alpha = lowalpha | hialpha 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" digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" safe = "$" | "-" | "_" | "." | "+" extra = "!" | "*" | "'" | "(" | ")" | "," national = "{" | "}" | "|" | "\" | "^" | "~" | "[" | "]" | "`" reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" punctuation = "<" | ">" | "#" | "%" | <">