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Note: The following is an excerpt from the official IETF RFC. It is provided here as a convenience and is not authoritative. Refer to the original document as the authoritative reference.

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 - Augmented BNF for Syntax Specifications: ABNF ].