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RFC 5234, "Augmented BNF for Syntax Specifications: ABNF", January 2008

Source of RFC: IETF - NON WORKING GROUP
Area Assignment: app

Errata ID: 2968
Status: Verified
Type: Technical

Reported By: Daniel van Vugt
Date Reported: 2011-09-12
Verifier Name: Pete Resnick
Date Verified: 2013-01-28

Section 4 says:

elements       =  alternation *c-wsp

It should say:

elements       =  alternation *WSP

Notes:

The grammar in section 4 of RFC 5234 is ambiguous. This was discovered by my own parsing code when trying to parse the ABNF grammar with itself. The ambiguity can be seen in a simplified form using the following 10 characters of input:

Input: X = Y \r \n ; Z \r \n
Offset: 0 1 2 3 4 5 6 7 8 9

My parser finds these two (ambiguous) solutions...

SOLUTION 1:

rulelist @ 0 len 10
rule @ 0 len 10
rulename @ 0 len 1 "X"
ALPHA @ 0 len 1
star_c_wsp @ 1 len 0
defined_as @ 1 len 1
star_c_wsp @ 2 len 0
elements @ 2 len 4
alternation @ 2 len 1
concatenation @ 2 len 1
repetition @ 2 len 1
element @ 2 len 1
rulename @ 2 len 1 "Y"
ALPHA @ 2 len 1
star_c_wsp @ 3 len 3
c_wsp @ 3 len 3
c_nl @ 3 len 2
CRLF @ 3 len 2
CR @ 3 len 1
LF @ 4 len 1
WSP @ 5 len 1
SP @ 5 len 1
c_nl @ 6 len 4
comment @ 6 len 4 ";Z
"
WSP_or_VCHAR @ 7 len 1
VCHAR @ 7 len 1
CRLF @ 8 len 2
CR @ 8 len 1
LF @ 9 len 1

SOLUTION 2:

rulelist @ 0 len 10
rule @ 0 len 5
rulename @ 0 len 1 "X"
ALPHA @ 0 len 1
star_c_wsp @ 1 len 0
defined_as @ 1 len 1
star_c_wsp @ 2 len 0
elements @ 2 len 1
alternation @ 2 len 1
concatenation @ 2 len 1
repetition @ 2 len 1
element @ 2 len 1
rulename @ 2 len 1 "Y"
ALPHA @ 2 len 1
star_c_wsp @ 3 len 0
c_nl @ 3 len 2
CRLF @ 3 len 2
CR @ 3 len 1
LF @ 4 len 1
star_c_wsp @ 5 len 1
c_wsp @ 5 len 1
WSP @ 5 len 1
SP @ 5 len 1
c_nl @ 6 len 4
comment @ 6 len 4 ";Z
"
WSP_or_VCHAR @ 7 len 1
VCHAR @ 7 len 1
CRLF @ 8 len 2
CR @ 8 len 1
LF @ 9 len 1


The solution to this ambiguity is to change:
elements = alternation *c-wsp
to:
elements = alternation *WSP


--VERIFIER NOTES--

The current document is clearly incorrect. However, though the solution appears correct, it has not been tested.

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