feat: support zero length folder name for pure conditional wrapper

lib/write-project/2-filter-by-features.js

@@ -1,6 +1,6 @@
 const applicable = require('../applicable');
 const path = require('path');
-const FILTERED_FILE = /^(.+)__if_(.+)$/;
+const FILTERED_FILE = /^(.*)__if_(.+)$/;
 
 // filter by features on some-file.ext__if_sass_or_less
 // support nested condition on every folder level
@@ -20,9 +20,15 @@ module.exports = function(features) {
         const cleanPart = match[1];
         const condition = match[2];
 
+        if (cleanPart === '' && i === ii - 1) {
+          throw new Error('cannot have an empty filename before __if_: ' + file.relative);
+        }
+
         if (applicable(features, condition)) {
           // Remove __if_x from the part.
-          cleanParts.push(cleanPart);
+          if (cleanPart !== '') {
+            cleanParts.push(cleanPart);
+          }
         } else {
           // Not applicable, skip this file
           return;

test/write-project/2-filter-by-features.spec.js

@@ -41,6 +41,16 @@ test('filterByFeatures rejects file failed condition', t => {
   t.falsy(newFile);
 });
 
+test('filterByFeatures rejects empty file name', t => {
+  const file = new Vinyl({
+    cwd: '/',
+    base: '/test/',
+    path: '/test/__if_c',
+    contents: Buffer.from('abc')
+  });
+  t.throws(() => filter(file));
+});
+
 test('filterByFeatures picks folder meets condition', t => {
   const file = new Vinyl({
     cwd: '/',
@@ -54,6 +64,19 @@ test('filterByFeatures picks folder meets condition', t => {
   t.is(newFile.contents.toString(), 'abc');
 });
 
+test('filterByFeatures supports zero length folder', t => {
+  const file = new Vinyl({
+    cwd: '/',
+    base: '/test/',
+    path: '/test/__if_b/file.any__if_a',
+    contents: Buffer.from('abc')
+  });
+  const newFile = filter(file);
+  t.is(newFile.basename, 'file.any');
+  t.is(newFile.path.replace(/\\/g, '/'), '/test/file.any');
+  t.is(newFile.contents.toString(), 'abc');
+});
+
 test('filterByFeatures rejects folder failed condition', t => {
   const file = new Vinyl({
     cwd: '/',