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General Internet Engineering Task Force BNF ABNF Syntax 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.

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 .

This document makes changes to two parts of . The two changes are as follows: Replace the last half of Section 2.3 of (beginning with "ABNF permits the specification of literal text strings") with the contents of . Replace the <char-val> rule in Section 4 of with the contents of .

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

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:

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:

and:

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

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. For example:

or

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.

Security is truly believed to be irrelevant to this document.