ruby 51 lines · 8 steps

Parsing user-agent strings in Ruby

A small class matches a user-agent string against ordered regex tables to extract browser, version, and OS.

Explained by highlit

Intermediate regex pattern-matching lookup-tables struct parsing

1class UserAgentParser
BROWSERS = [
  [/Edg\/([\d.]+)/, "Edge"],
  [/OPR\/([\d.]+)/, "Opera"],
  [/Chrome\/([\d.]+)/, "Chrome"],
  [/Version\/([\d.]+).*Safari/, "Safari"],
  [/Firefox\/([\d.]+)/, "Firefox"],
  [/MSIE ([\d.]+)/, "Internet Explorer"]
].freeze
10 
OPERATING_SYSTEMS = [
  [/Windows NT 10\.0/, "Windows 10"],
  [/Windows NT 6\.3/, "Windows 8.1"],
  [/Mac OS X ([\d_]+)/, "macOS"],
  [/Android ([\d.]+)/, "Android"],
  [/iPhone OS ([\d_]+)/, "iOS"],
  [/Linux/, "Linux"]
].freeze
19 
Result = Struct.new(:browser, :browser_version, :os, keyword_init: true)
21 
def self.parse(user_agent)
  new(user_agent.to_s).parse
end
25 
def initialize(user_agent)
  @user_agent = user_agent
end
29 
def parse
  browser, version = match_first(BROWSERS)
  os, = match_first(OPERATING_SYSTEMS)
33 
  Result.new(
    browser: browser || "Unknown",
    browser_version: version,
    os: os || "Unknown"
  )
end
40 
private
42 
def match_first(patterns)
  patterns.each do |regexp, label|
    if (m = @user_agent.match(regexp))
      return [label, m[1]&.tr("_", ".")]
    end
  end
  [nil, nil]
end
51end

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Ordering your patterns most-specific-first avoids false matches when strings overlap, like Edge and Opera masquerading as Chrome.
2Returning a Struct instead of a raw hash gives callers named, self-documenting fields with almost no boilerplate.
3A single reusable matcher method keeps browser and OS extraction DRY by treating both as the same ordered-table problem.

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