Allow custom interpolation patterns for placeholders

lib/i18n/config.rb

@@ -136,5 +136,18 @@ def enforce_available_locales
     def enforce_available_locales=(enforce_available_locales)
       @@enforce_available_locales = enforce_available_locales
     end
+
+    # Allows clients to provide custom placeholders.
+    #
+    # E.g. using {{}} as a placeholder: "{{hello}}, world!"
+    # I18n.config.interpolation_patterns << /\{\{(\w+)\}\}/
+    def interpolation_patterns
+      @@interpolation_patterns ||= I18n::INTERPOLATION_PATTERNS
+    end
+
+    # Sets interpolation patterns. Expect an array of patterns.
+    def interpolation_patterns=(interpolation_patterns)
+      @@interpolation_patterns = interpolation_patterns
+    end
   end
 end

lib/i18n/interpolate/ruby.rb

@@ -2,11 +2,12 @@
 # https://github.com/mutoh/gettext/blob/f6566738b981fe0952548c421042ad1e0cdfb31e/lib/gettext/core_ext/string.rb
 
 module I18n
-  INTERPOLATION_PATTERN = Regexp.union(
+  INTERPOLATION_PATTERNS = [
     /%%/,
     /%\{(\w+)\}/,                               # matches placeholders like "%{foo}"
     /%<(\w+)>(.*?\d*\.?\d*[bBdiouxXeEfgGcps])/  # matches placeholders like "%<foo>.d"
-  )
+  ]
+  INTERPOLATION_PATTERN = Regexp.union(INTERPOLATION_PATTERNS)
 
   class << self
     # Return String or raises MissingInterpolationArgument exception.
@@ -18,18 +19,20 @@ def interpolate(string, values)
     end
 
     def interpolate_hash(string, values)
-      string.gsub(INTERPOLATION_PATTERN) do |match|
-        if match == '%%'
+      pattern = Regexp.union(config.interpolation_patterns)
+      string.gsub(pattern) do |match_str|
+        if match_str == '%%'
           '%'
         else
-          key = ($1 || $2).to_sym
+          placeholder, format = match_str.match(pattern).captures.reject(&:nil?)
+          key = placeholder.to_sym
           value = if values.key?(key)
                     values[key]
                   else
                     config.missing_interpolation_argument_handler.call(key, values, string)
                   end
           value = value.call(values) if value.respond_to?(:call)
-          $3 ? sprintf("%#{$3}", value) : value
+          format ? sprintf("%#{format}", value) : value
         end
       end
     end

test/i18n/interpolate_test.rb

@@ -78,3 +78,20 @@ def teardown
       I18n.interpolate("%{first} %{last}", :first => 'Masao')
   end
 end
+
+class I18nCustomInterpolationPatternTest < I18n::TestCase
+  def setup
+    super
+    @old_interpolation_patterns = I18n.config.interpolation_patterns
+    I18n.config.interpolation_patterns << /\{\{(\w+)\}\}/
+  end
+
+  def teardown
+    I18n.config.interpolation_patterns = @old_interpolation_patterns
+    super
+  end
+
+  test "String interpolation can use custom interpolation pattern" do
+    assert_equal "Masao Mutoh", I18n.interpolate("{{first}} {{last}}", :first => 'Masao', :last => 'Mutoh')
+  end
+end