Optimizer: remove non-capturing empty groups

src/optimizer/transforms/__tests__/remove-empty-group-transform-test.js

@@ -0,0 +1,39 @@
+/**
+ * The MIT License (MIT)
+ * Copyright (c) 2017-present Dmitry Soshnikov <[email protected]>
+ */
+
+'use strict';
+
+const {transform} = require('../../../transform');
+const removeEmptyGroup = require('../remove-empty-group-transform');
+
+describe('remove empty groups', () => {
+
+  it('removes empty groups', () => {
+    const re = transform(/a(?:)b/, [
+      removeEmptyGroup
+    ]);
+    expect(re.toString()).toBe('/ab/');
+
+    const re2 = transform(/((?:))/, [
+      removeEmptyGroup
+    ]);
+    expect(re2.toString()).toBe('/()/');
+  });
+
+  it('does not remove empty regexp', () => {
+    const re = transform(/(?:)/, [
+      removeEmptyGroup
+    ]);
+    expect(re.toString()).toBe('/(?:)/');
+  });
+
+  it('removes empty group quantifier', () => {
+    const re = transform(/(?:)+/, [
+      removeEmptyGroup
+    ]);
+    expect(re.toString()).toBe('/(?:)/');
+  });
+
+});

src/optimizer/transforms/index.js

@@ -30,6 +30,9 @@ module.exports = [
   // (a|b|c) -> [abc]
   require('./group-single-chars-to-char-class'),
 
+  // (?:)a -> a
+  require('./remove-empty-group-transform'),
+
   // (?:a) -> a
   require('./ungroup-transform')
 ];

src/optimizer/transforms/remove-empty-group-transform.js

@@ -0,0 +1,34 @@
+/**
+ * The MIT License (MIT)
+ * Copyright (c) 2017-present Dmitry Soshnikov <[email protected]>
+ */
+
+'use strict';
+
+/**
+ * A regexp-tree plugin to remove non-capturing empty groups.
+ *
+ * /(?:)a/ -> /a/
+ * /a|(?:)/ -> /a|/
+ */
+
+module.exports = {
+  Group(path) {
+    const {node, parent} = path;
+    const childPath = path.getChild();
+
+    if (node.capturing || childPath) {
+      return;
+    }
+
+    if (parent.type === 'Repetition') {
+
+      path.getParent().replace(node);
+
+    } else if (parent.type !== 'RegExp') {
+
+      path.remove();
+
+    }
+  }
+};