[REBOL] hostname parse rule Re:(4)
Hi /PeO ... These were the rules I was using... let-char: charset[#"a" - #"z" #"A" - #"Z"] let-digit-char: charset[#"a" - #"z" #"A" - #"Z" #"0" - #"9"] let-digit-hyph-char: charset[#"a" - #"z" #"A" - #"Z" #"0" - #"9" #"-"] The problem is combining them to a working ruleset... The problem is how you formulate your specification. You say: ; may have any number of letters, digits and ; hyphens in between, but this sequence must ; end with a letter or digit if it exists This is too general. You need to redefine your rules so that you precisely state that a hyphen - if it occurs - must be followed by a letter or digit. I.e., I believe you have a let-digit-char rule, your let-digit-hyph-char rule then must be: The hyphen must not be followed by a letter or a digit unless it is the first of the two last characters.. According to the RFC "a--a" should be a valid host name (part)... BNF rule was.. (not retyped from the RFC, but what I want to implement as a parse rule).. let-char [*[let-digit-hyph-char] let-digit-char] with "[" and "]" marking optional parts as in BNF That is.. valid host names begins with a letter, ends with a letter or a digit (which could be the same as the first letter, since one-character host names are allowed), in between there could be any number of letters, digits and hyphens in any order. let-digit-hyph-char: [ "-" let-digit-char] That rule will require every hyphen to be followed by a letter or a digit.. This rule only evaluates to true if a hyphen is immediately followed by a character. It permits that a hyphen appear, provided that the hyphen is followed by a an obligatory character and thereby excludes the possibility of a trailing hyphen. Hope this helps ;- Elan [: - )]
[REBOL] hostname parse rule Re:
Hi there,
This might be a good chance to plug my regular expression emulator,
search-text.r :)
http://www.rebol.org/utility/search-text.r
As far as I understand it, this should be the direct translation of the
BNF rule for hostnames as given by /PeO :
let-char [*[let-digit-hyph-char] let-digit-char]
should be:
l: to-bits #!a-z; TO-BITS is included in search-text.r
== make bitset! #{ ; ! means upper and lower case
FE07FE07
}
ld: to-bits #!a-z0-9
== make bitset! #{
FF03FE07FE07
}
ldh: to-bits #!a-z0-9\- ; \ escapes the hyphen
== make bitset! #{
0020FF03FE07FE07
}
name-rule: [l 0 1 [any ldh ld]]
== [l 0 1 [any ldh ld]]
Unfortunately this almost never returns true when it should:
parse/all "abc-ef" name-rule
== false
parse/all "abcef" name-rule
== false
parse/all "a" name-rule
== true
This is very easy to do with SEARCH from search-text.r, and you don't
even have to prepare the bitsets beforehand:
search "abcdef" [head #!a-z maybe [any #!a-z0-9\- #!a-z] tail]
== [1 6 "abcdef"]
search "abc-def" [head #!a-z maybe [any #!a-z0-9\- #!a-z] tail]
== [1 7 "abc-def"]
search "abc--def" [head #!a-z maybe [any #!a-z0-9\- #!a-z] tail]
== [1 8 "abc--def"]
search "abcdef-" [head #!a-z maybe [any #!a-z0-9\- #!a-z] tail]
== none
search "0abcdef" [head #!a-z maybe [any #!a-z0-9\- #!a-z] tail]
== none
SEARCH emulates the backtracking behavior of regular expressions.
PARSE on the other hand will match to the end of the line with
LET-DIGIT-HYPH-CHAR, leaving nothing for the last LET-DIGIT-CHAR to
match.
Actually, I have to admit this isn't so difficult with PARSE either.
You just have to look for sequences of any number of optional hyphens
followed by one or more alphanumerics:
name-rule: [l any [any "-" some ld ]]
== [l any [any "-" ld]]
parse/all "abcdef" name-rule
== true
parse/all "abc-def" name-rule
== true
parse/all "abc--def" name-rule
== true
parse/all "abcdef-" name-rule
== false
parse/all "0abcdef" name-rule
== false
See you,
Eric
[REBOL] hostname parse rule Re:(2)
name-rule: [let-char [none | [[some let-digit-hyph-char] let-digit-char]] to end] Try: let-char any [let-digit-hyp=char] instead. The problem will be that it would then accept a "-" as the last character in a part of the hostname... Valid names are these starting with a letter, followed by an optional part consisting of any number of letters and "-":es, but must end with a letter or digit... The problem with my original rule was that the [some let-digit-hyph-char] part matched all the way up to the end of the string that I tried to parse.. (some should be "any", but that didn't help) I came up with this rule that seems to be working: name-rule: [ let-char [[any [[let-digit-hyph-char] let-digit-char]] | none]] parse "a" name-rule == true parse "1" name-rule == false parse "a-" name-rule == false parse "a-1" name-rule == true parse "aaa-1" name-rule == true /PeO
[REBOL] hostname parse rule Re:(2)
Hello [EMAIL PROTECTED]! On 13-Feb-00, you wrote: p hname-rule: [name-rule some ["." name-rule]] name-rule: p [let-char [none | [[some let-digit-hyph-char] let-digit-char]] p to end] p The problem is that it returns "true" too often... This is caused by the "to end" above. It should work if you remove it. Removing the "to end" did break the rule even more... name-rule: [let-char [none | [[some let-digit-hyph-char] let-digit-char]]] parse "a" name-rule == true OK a single letter hostname part is ok parse "a1" name-rule == false ERR!two chars which the first one is a letter and the last is letter or digit is ok parse "a1a" name-rule == false ERR!should be ok replacing "some" with "any" above caused the same error.. And.. the rule I came up with that I posted earlier... name-rule: [ let-char [[any [[let-digit-hyph-char] let-digit-char]] | none]] didn't work with a two character name... If I break down the rules... ; name must start with a letter let-char ; may have any number of letters, digits and ; hyphens in between, but this sequence must ; end with a letter or digit if it exists 0 1 [any let-digit-hyph-char let-digit-char] The problem is combining them to a working ruleset... parse "a" name-rule should return "true" parse "a1" name-rule should return "true" parse "aa" name-rule should return "true" parse "a-" name-rule should return "false" parse "a-1" name-rule should return "true" /PeO
[REBOL] hostname parse rule Re:(3)
Hi /PeO you wrote: ; name must start with a letter let-char ; may have any number of letters, digits and ; hyphens in between, but this sequence must ; end with a letter or digit if it exists 0 1 [any let-digit-hyph-char let-digit-char] The problem is combining them to a working ruleset... The problem is how you formulate your specification. You say: ; may have any number of letters, digits and ; hyphens in between, but this sequence must ; end with a letter or digit if it exists This is too general. You need to redefine your rules so that you precisely state that a hyphen - if it occurs - must be followed by a letter or digit. I.e., I believe you have a let-digit-char rule, your let-digit-hyph-char rule then must be: let-digit-hyph-char: [ "-" let-digit-char] This rule only evaluates to true if a hyphen is immediately followed by a character. It permits that a hyphen appear, provided that the hyphen is followed by a an obligatory character and thereby excludes the possibility of a trailing hyphen. Hope this helps ;- Elan [: - )]
[REBOL] hostname parse rule Re:
Hi [EMAIL PROTECTED] you wrote: let-char: charset[#"a" - #"z" #"A" - #"Z"] let-digit-char: charset[#"a" - #"z" #"A" - #"Z" #"0" - #"9"] let-digit-hyph-char: charset[#"a" - #"z" #"A" - #"Z" #"0" - #"9" #"-"] hname-rule: [name-rule some ["." name-rule]] name-rule: [let-char [none | [[some let-digit-hyph-char] let-digit-char]] to end] parse "asdfg-" name-rule == true ERR!last character is not one of the allowed ones parse "a%$#" name-rule == true ERR!none of the characters following the "a" are allowed What is wrong with the rule generating the too-many true's ? Your name-rule requres a let-char then it permits for none and then goes to end. In both ERR! cases let-char is fulfilled, then at some point the none part of the following bracket is fulfilled and then to end is performed. No wonder you end up with true in both cases. Compare to: parse "abcdefg" [none to end] == true ;- Elan [: - )]
[REBOL] hostname parse rule Re:
name-rule: [let-char [none | [[some let-digit-hyph-char] let-digit-char]] to end] Try: let-char any [let-digit-hyp=char] instead. Andrew Martin ICQ: 26227169 http://members.xoom.com/AndrewMartin/ --
