Case-Sensitive String Support in ABNF
RFC 7405

Document Type RFC - Proposed Standard (December 2014; Errata)
Updates RFC 5234
Was draft-kyzivat-case-sensitive-abnf (individual in app area)
Author Paul Kyzivat 
Last updated 2018-04-26
Stream Internet Engineering Task Force (IETF)
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Internet Engineering Task Force (IETF)                        P. Kyzivat
Request for Comments: 7405                                 December 2014
Updates: 5234
Category: Standards Track
ISSN: 2070-1721

                 Case-Sensitive String Support in ABNF


   This document extends the base definition of ABNF (Augmented Backus-
   Naur Form) to include a way to specify US-ASCII string literals that
   are matched in a case-sensitive manner.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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RFC 7405          Case-Sensitive String Support in ABNF    December 2014

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Updates to RFC 5234 . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Terminal Values - Literal Text Strings  . . . . . . . . .   3
     2.2.  ABNF Definition of ABNF - char-val  . . . . . . . . . . .   4
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   4.  Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Introduction

   The base definition of ABNF (Augmented Backus-Naur Form) supports US-
   ASCII string literals.  The matching of these literals is done in a
   case-insensitive manner.  While this is often the desired behavior,
   in some situations, case-sensitive matching of string literals is
   needed.  Literals for case-sensitive matching must be specified using
   the numeric representation of those characters, which is inconvenient
   and error prone both to write and read.

   This document extends ABNF to have two different types of US-ASCII
   string literals.  One type is matched using case-sensitive matching,
   while the other is matched using case-insensitive matching.  These
   types are denoted using type prefixes similar to the type prefixes
   used with numeric values.  If no prefix is used, then case-
   insensitive matching is used (as is consistent with previous

   This document is structured as a set of changes to the full ABNF
   specification [RFC5234].

2.  Updates to RFC 5234

   This document makes changes to two parts of [RFC5234].  The two
   changes are as follows:

   o  Replace the last half of Section 2.3 of [RFC5234] (beginning with
      "ABNF permits the specification of literal text strings") with the
      contents of Section 2.1.

   o  Replace the <char-val> rule in Section 4 of [RFC5234] with the
      contents of Section 2.2.

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RFC 7405          Case-Sensitive String Support in ABNF    December 2014

2.1.  Terminal Values - Literal Text Strings

   ABNF permits the specification of literal text strings directly,
   enclosed in quotation marks.  Hence:

         command     =  "command string"

   Literal text strings are interpreted as a concatenated set of
   printable characters.


   The character set for these strings is US-ASCII.

   Literal text strings in ABNF may be either case sensitive or case
   insensitive.  The form of matching used with a literal text string is
   denoted by a prefix to the quoted string.  The following prefixes are

         %s          =  case-sensitive
         %i          =  case-insensitive

   To be consistent with prior implementations of ABNF, having no prefix
   means that the string is case insensitive and is equivalent to having
   the "%i" prefix.


         rulename = %i"aBc"


         rulename = "abc"

   will both match "abc", "Abc", "aBc", "abC", "ABc", "aBC", "AbC", and

   In contrast:

         rulename = %s"aBc"

   will match only "aBc" and will not match "abc", "Abc", "abC", "ABc",
   "aBC", "AbC", or "ABC".

   In the past, the numerical specification of individual characters was
   used to define a case-sensitive rule.

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   For example:

         rulename    =  %d97 %d98 %d99


         rulename    =  %x61.62.63

   will match only the string that comprises only the lowercase
   characters, abc.  Using a literal text string with a prefix has a
   clear readability advantage over the old way.

2.2.  ABNF Definition of ABNF - char-val

         char-val       =  case-insensitive-string /

         case-insensitive-string =
                           [ "%i" ] quoted-string

         case-sensitive-string =
                           "%s" quoted-string

         quoted-string  =  DQUOTE *(%x20-21 / %x23-7E) DQUOTE
                                ; quoted string of SP and VCHAR
                                ;  without DQUOTE

3.  Security Considerations

   Security is truly believed to be irrelevant to this document.

4.  Normative References

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008,

Author's Address

   Paul Kyzivat
   United States


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