Allow to pass regular expressions to buildRegExps

Summary: Allows to use `setupBabel.buildRegExps` with regular expressions, not only with strings.

Reviewed By: mjesun

Differential Revision: D5614738

fbshipit-source-id: ccad4d49f8d2006a714833989b3c6be2ce7071ab

setupBabel.js

@@ -13,9 +13,7 @@ const babelRegisterOnly = require('metro-bundler/src/babelRegisterOnly');
 const escapeRegExp = require('lodash/escapeRegExp');
 const path = require('path');
 
-const BABEL_ENABLED_PATHS = [
-  'local-cli',
-];
+const BABEL_ENABLED_PATHS = ['local-cli'];
 
 /**
  * We use absolute paths for matching only the top-level folders reliably. For
@@ -23,10 +21,22 @@ const BABEL_ENABLED_PATHS = [
  * have the same name as one of `BABEL_ENABLED_PATHS`.
  */
 function buildRegExps(basePath, dirPaths) {
-  return dirPaths.map(folderPath =>
-    // Babel `only` option works with forward slashes in the RegExp so replace
-    // backslashes for Windows.
-    new RegExp(`^${escapeRegExp(path.resolve(basePath, folderPath).replace(/\\/g, '/'))}`)
+  return dirPaths.map(
+    folderPath =>
+      // Babel `only` option works with forward slashes in the RegExp so replace
+      // backslashes for Windows.
+      folderPath instanceof RegExp
+        ? new RegExp(
+            `^${escapeRegExp(
+              path.resolve(basePath, '.').replace(/\\/g, '/'),
+            )}/${folderPath.source}`,
+            folderPath.flags,
+          )
+        : new RegExp(
+            `^${escapeRegExp(
+              path.resolve(basePath, folderPath).replace(/\\/g, '/'),
+            )}`,
+          ),
   );
 }