Add method to create machine tag tag objects.

lib/machine_tag/tag.rb

@@ -2,10 +2,12 @@ module MachineTag
   # A tag which can be a machine tag.
   #
   class Tag < String
+    PREFIX = /[a-z][a-z0-9_]*/i
+
     # The regular expression for matching a machine tag
     #
     # @return [Regexp] the regular expression for matching a machine tag
-    MACHINE_TAG = /^(?<namespace>[a-zA-Z][a-zA-Z0-9_]*):(?<predicate>[a-zA-Z][a-zA-Z0-9_]*)=(?<value>.*)$/
+    MACHINE_TAG = /^(?<namespace>#{PREFIX}):(?<predicate>#{PREFIX})=(?<value>.*)$/
 
     # The namespace portion of the machine tag, +nil+ if the tag is not a machine tag
     #
@@ -39,6 +41,30 @@ module MachineTag
       end
     end
 
+    # Creates a machine tag from a given namespace, predicate, and value.
+    #
+    # @param namespace [String] the namespace
+    # @param predicate [String] the predicate
+    # @param value [String, Array, #to_s] the value
+    #
+    # @option options [String] :separator (',') the separator to use when +value+ is an +Array+
+    #
+    # @return [Tag] the machine tag
+    #
+    def self.machine_tag(namespace, predicate, value, options = {})
+      raise ArgumentError, "Invalid machine tag namespace: #{namespace.inspect}" unless namespace =~ /^#{PREFIX}$/
+      raise ArgumentError, "Invalid machine tag predicate: #{predicate.inspect}" unless predicate =~ /^#{PREFIX}$/
+
+      options[:separator] ||= ','
+
+      case value
+      when Array
+        new("#{namespace}:#{predicate}=#{value.join(options[:separator])}")
+      else
+        new("#{namespace}:#{predicate}=#{value}")
+      end
+    end
+
     # Returns whether this tag is a machine tag or not.
     #
     # @return [Boolean] +true+ if this tag is a machine tag, otherwise +false+