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Welcome to the AcceptLanguage wiki! This gem helps you parse the Accept-Language HTTP header and match user language preferences against your application's available locales.

Table of Contents What is Accept-Language? Quick Example Key Features Wiki Pages Getting Started Core Concepts Framework Integration Practical Usage Standards Reference Additional Resources Contributing

What is Accept-Language?

When a browser sends a request to your server, it includes an Accept-Language header that looks something like this:

Accept-Language: fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5

This header tells your application: "I prefer Swiss French, then French, then English, then German, and I'll accept anything else as a last resort."

Parsing this header correctly—respecting quality values, wildcards, and prefix matching rules—is exactly what this gem does.

Quick Example

require "accept_language"

# Parse the header and find the best match
AcceptLanguage.parse("fr-CH, fr;q=0.9, en;q=0.8").match(:en, :fr, :"fr-CH")
# => :"fr-CH"

That's it. One line to parse, one method to match.

Key Features

Standards-compliant

Implements RFC 7231 (Accept-Language header) and RFC 4647 (Basic Filtering matching scheme).

Quality values

Respects user preferences from q=1 (most preferred) down to q=0 (not acceptable).

Prefix matching

The range en correctly matches en-US, en-GB, and other regional variants.

Wildcards

Supports * to match any language not explicitly listed.

Exclusions

Handles q=0 to explicitly reject specific languages.

Thread-safe

Parser instances are immutable and safe to share across threads.

Wiki Pages

Getting Started

• Getting Started — Install the gem and write your first matching code in 5 minutes.

Core Concepts

• Understanding Quality Values — Learn how q=0.8 differs from q=1 and why it matters.
• Basic Filtering Explained — Understand how en matches en-GB but not eng.
• Working with Wildcards — Use * to accept any language as a fallback.
• Excluding Languages — Reject specific languages with q=0.
• BCP 47 Language Tags — Scripts, regions, and variants demystified.

Framework Integration

• Rails Integration Guide — Set up automatic locale detection in Ruby on Rails.
• Rack Integration Guide — Build middleware for Sinatra, Hanami, or pure Rack apps.

Practical Usage

• Real-World Examples — Patterns for e-commerce, SaaS, APIs, and more.

Standards Reference

This library conforms to:

Specification	Description
RFC 7231 §5.3.5	Accept-Language header definition
RFC 7231 §5.3.1	Quality values syntax
RFC 4647 §3.3.1	Basic Filtering matching scheme
BCP 47	Language tag structure

Additional Resources

• GitHub Repository
• API Documentation
• RubyGems Page

Contributing

Found an issue or have a suggestion? Feel free to open an issue or submit a pull request.

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Ready to get started? Head over to Getting Started →