Case-Sensitive String Support in ABNF
draft-kyzivat-case-sensitive-abnf-02

Abstract

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

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Table of Contents

• 1. Introduction
• 2. Updates to RFC5234
• 2.1. Terminal values - literal text strings
• 2.2. ABNF Definition of ABNF - char-val
• 3. IANA Considerations
• 4. Security Considerations
• 5. Normative References
• Author's Address

1. Introduction

The base definition of ABNF (Augmented Backus-Naur Form) supports ASCII string literals. 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. That is inconvenient and error prone both to write and to read.

This document extends ABNF to have two different types of 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, consistent with previous behavior.

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

2. Updates to RFC5234

This document makes changes to two parts of RFC5234. The two changes are:

• 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 below.
• Replace the <char-val> rule in section 4 of RFC5234 with the contents of Section 2.2 below.

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 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".

The way that has been used in the past to define a rule that is case sensitive is to specify the individual characters numerically.

For example:

      rulename    =  %d97 %d98 %d99
        

or

      rulename    =  %x61.62.63
        

will match only the string that comprises only the lowercase characters, abc. The new way (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. IANA Considerations

This memo includes no request to IANA.

4. Security Considerations

Security is truly believed to be irrelevant to this document.

5. Normative References

Author's Address