diff --git a/Library/Homebrew/livecheck/strategy.rb b/Library/Homebrew/livecheck/strategy.rb
index 32cd9b7822..45b671defd 100644
--- a/Library/Homebrew/livecheck/strategy.rb
+++ b/Library/Homebrew/livecheck/strategy.rb
@@ -111,7 +111,7 @@ module Homebrew
           constant = Strategy.const_get(const_symbol)
           next unless constant.is_a?(Class)
 
-          key = const_symbol.to_s.underscore.to_sym
+          key = Utils.underscore(const_symbol).to_sym
           @strategies[key] = constant
         end
         @strategies
diff --git a/Library/Homebrew/test/utils_spec.rb b/Library/Homebrew/test/utils_spec.rb
index c75d21bed8..50dcb15228 100644
--- a/Library/Homebrew/test/utils_spec.rb
+++ b/Library/Homebrew/test/utils_spec.rb
@@ -58,4 +58,39 @@ describe Utils do
       expect(described_class.pluralize("foo", 2, singular: "o", plural: "es")).to eq("fooes")
     end
   end
+
+  describe ".underscore" do
+    # commented out entries require acronyms inflections
+    let (:words) {
+      [["API", "api"],
+      ["APIController", "api_controller"],
+      ["Nokogiri::HTML", "nokogiri/html"],
+      # ["HTTPAPI", "http_api"],
+      ["HTTP::Get", "http/get"],
+      ["SSLError", "ssl_error"],
+      # ["RESTful", "restful"],
+      # ["RESTfulController", "restful_controller"],
+      # ["Nested::RESTful", "nested/restful"],
+      # ["IHeartW3C", "i_heart_w3c"],
+      # ["PhDRequired", "phd_required"],
+      # ["IRoRU", "i_ror_u"],
+      # ["RESTfulHTTPAPI", "restful_http_api"],
+      # ["HTTP::RESTful", "http/restful"],
+      # ["HTTP::RESTfulAPI", "http/restful_api"],
+      # ["APIRESTful", "api_restful"],
+      ["Capistrano", "capistrano"],
+      ["CapiController", "capi_controller"],
+      ["HttpsApis", "https_apis"],
+      ["Html5", "html5"],
+      ["Restfully", "restfully"],
+      ["RoRails", "ro_rails"]]
+    }
+
+    it "converts strings to underscore case" do
+      words.each do |camel, under|
+        expect(described_class.underscore(camel)).to eq(under)
+        expect(described_class.underscore(under)).to eq(under)
+      end
+    end
+  end
 end
diff --git a/Library/Homebrew/utils.rb b/Library/Homebrew/utils.rb
index c0eaae9b41..1880aee7d5 100644
--- a/Library/Homebrew/utils.rb
+++ b/Library/Homebrew/utils.rb
@@ -131,4 +131,23 @@ module Utils
     suffix = (count == 1) ? singular : plural
     "#{stem}#{suffix}"
   end
+
+  # Makes an underscored, lowercase form from the expression in the string.
+  #
+  # Changes '::' to '/' to convert namespaces to paths.
+  #
+  #   underscore('ActiveModel')         # => "active_model"
+  #   underscore('ActiveModel::Errors') # => "active_model/errors"
+  #
+  # @see https://github.com/rails/rails/blob/v6.1.7.2/activesupport/lib/active_support/inflector/methods.rb#L81-L100
+  #   `ActiveSupport::Inflector.underscore`
+  sig { params(camel_cased_word: T.any(String, Symbol)).returns(String) }
+  def self.underscore(camel_cased_word)
+    return camel_cased_word unless /[A-Z-]|::/.match?(camel_cased_word)
+    word = camel_cased_word.to_s.gsub("::", "/")
+    word.gsub!(/([A-Z])(?=[A-Z][a-z])|([a-z\d])(?=[A-Z])/) { ($1 || $2) << "_" }
+    word.tr!("-", "_")
+    word.downcase!
+    word
+  end
 end