Improve escaping path separators for regexp on Windows (#6523)

Author: hron

Diff for: CHANGELOG.md

@@ -4,6 +4,10 @@
 
 - `[jest-snapshot]` Introduce `toMatchInlineSnapshot` and `toThrowErrorMatchingInlineSnapshot` matchers ([#6380](https://github.com/facebook/jest/pull/6380))
 
+### Fixes
+
+- `[jest-regex-util]` Improve handling already escaped path separators on Windows ([#6523](https://github.com/facebook/jest/pull/6523))
+
 ### Chore & Maintenance
 
 - `[website]` Switch domain to https://jestjs.io ([#6549](https://github.com/facebook/jest/pull/6549))

Diff for: packages/jest-regex-util/src/__tests__/index.test.js

@@ -31,17 +31,23 @@ describe('replacePathSepForRegex()', () => {
 
     it('should not escape an escaped dot', () => {
       expect(replacePathSepForRegex('a\\.dotfile')).toBe('a\\.dotfile');
-
-      // If we expect Windows path separators to be escaped, one would expect
-      // the regular expression "\\\." to be unescaped as "\.". This is not the
-      // current behavior.
-      expect(replacePathSepForRegex('a\\\\\\.dotfile')).toBe(
-        'a\\\\\\\\\\.dotfile',
-      );
+      expect(replacePathSepForRegex('a\\\\\\.dotfile')).toBe('a\\\\\\.dotfile');
     });
 
     it('should not escape an escaped regexp symbol', () => {
       expect(replacePathSepForRegex('b\\(86')).toBe('b\\(86');
     });
+
+    it('should escape Windows path separators inside groups', () => {
+      expect(replacePathSepForRegex('[/\\\\]')).toBe('[\\\\\\\\]');
+    });
+
+    it('should escape Windows path separator at the beginning', () => {
+      expect(replacePathSepForRegex('\\a')).toBe('\\\\a');
+    });
+
+    it('should not escape several already escaped path separators', () => {
+      expect(replacePathSepForRegex('\\\\\\\\')).toBe('\\\\\\\\');
+    });
   });
 });

Diff for: packages/jest-regex-util/src/index.js

@@ -23,7 +23,10 @@ export const escapeStrForRegex = (string: string) =>
 
 export const replacePathSepForRegex = (string: string) => {
   if (path.sep === '\\') {
-    return string.replace(/(\/|\\(?![[\]{}()*+?.^$|]))/g, '\\\\');
+    return string.replace(
+      /(\/|(.)?\\(?![[\]{}()*+?.^$|\\]))/g,
+      (_match, p1, p2) => (p2 && p2 !== '\\' ? p2 + '\\\\' : '\\\\'),
+    );
   }
   return string;
 };