Internet Engineering Task Force (IETF) P. Kyzivat
Request for Comments: 7405 December 2014
Category: Standards Track
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 22. Updates to RFC 5234 . . . . . . . . . . . . . . . . . . . . . 22.1. Terminal Values - Literal Text Strings . . . . . . . . . 32.2. ABNF Definition of ABNF - char-val . . . . . . . . . . . 43. Security Considerations . . . . . . . . . . . . . . . . . . . 44. Normative References . . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 41. 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
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.
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
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
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.
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 /
[ "%i" ] 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,