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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

Abstract

 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
 http://www.rfc-editor.org/info/rfc7405.

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
 (http://trustee.ietf.org/license-info) 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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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
 behavior).

 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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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.

 NOTE:

 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
 allowed:

       %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.

 Hence:

       rulename = %i"aBc"

 and:

       rulename = "abc"

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

 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

 or

       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-sensitive-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,
            <http:/www.rfc-editor.org/info/rfc5234>.

Author's Address

 Paul Kyzivat
 Massachusetts
 United States

 EMail: pkyzivat@alum.mit.edu

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