test: add regex feature tests

tests/features.rs

@@ -24,5 +24,5 @@ fn assert_output(world: &mut TestWorld, expected: String) {
 }
 
 fn main() {
-    futures::executor::block_on(TestWorld::run("tests/features/basic.feature"));
+    futures::executor::block_on(TestWorld::run("tests/features"));
 }

tests/features/regex.feature

@@ -0,0 +1,19 @@
+Feature: Regex search and replace
+
+  Scenario: You can search and replace with with a regular expression
+    Given Search is '(\w+)'
+    And Replace is 'new'
+    And Input is 'This is a'
+    Then Output is 'new new new'
+
+  Scenario: You can use a '$' to replace a match group
+    Given Search is 'function (\w+)\(\)'
+    And Replace is 'fun $1()'
+    And Input is 'function foo()'
+    Then Output is 'fun foo()'
+
+  Scenario: You can will need to wrap the match group when the match is against another word
+    Given Search is 'Hello (\w+)'
+    And Replace is 'Hello ${1}s'
+    And Input is 'Hello world'
+    Then Output is 'Hello worlds'