Correct regexp mutator in case of interpolation

Closes #67

lib/mutant/mutator/node/literal/regex.rb

@@ -10,10 +10,18 @@ module Mutant
           # No input can ever be matched with this
           NULL_REGEXP_SOURCE = 'a\A'.freeze
 
-          children :source, :options
-
         private
 
+          # Return options
+          #
+          # @return [Parser::AST::Node]
+          #
+          # @api private
+          #
+          def options
+            children.last
+          end
+
           # Emit mutants
           #
           # @return [undefined]
@@ -22,6 +30,9 @@ module Mutant
           #
           def dispatch
             emit_nil
+            children.each_with_index do |child, index|
+              mutate_child(index) unless child.type == :str
+            end
             emit_self(s(:str, EMPTY_STRING), options)
             emit_self(s(:str, NULL_REGEXP_SOURCE), options)
           end

spec/unit/mutant/mutator/node/literal/regex_spec.rb

@@ -2,14 +2,31 @@ require 'spec_helper'
 
 describe Mutant::Mutator::Node::Literal, 'regex' do
 
-  let(:source) { '/foo/' }
+  context 'literal' do
+    let(:source) { '/foo/' }
 
-  let(:mutations) do
-    mutations = []
-    mutations << 'nil'
-    mutations << '//' # match all
-    mutations << '/a\A/' # match nothing
+    let(:mutations) do
+      mutations = []
+      mutations << 'nil'
+      mutations << '//' # match all
+      mutations << '/a\A/' # match nothing
+    end
+
+    it_should_behave_like 'a mutator'
+  end
+
+  context 'interpolated' do
+    let(:source) { '/#{foo.bar}n/' }
+
+    let(:mutations) do
+      mutations = []
+      mutations << 'nil'
+      mutations << '//' # match all
+      mutations << '/#{foo}n/' # match all
+      mutations << '/a\A/' # match nothing
+    end
+
+    it_should_behave_like 'a mutator'
   end
 
-  it_should_behave_like 'a mutator'
 end