Fix --testPathPattern escaping for '\\' on Windows (#5230)

PR #5054 introduced a regression when handling escaped Windows path
separators in the `--testPathPattern`. The PR applied the same escaping
as the "Watch Usage" prompt, which incorrectly escapes `path\\.*file` as
`path\\\.*file`.

This commit fixes the regular expression used in "Watch Usage" and the
`--testPathPattern` CLI argument with unit tests.

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 * `[jest-config]` fix unexpected condition to avoid infinite recursion in
   Windows platform. ([#5161](https://github.com/facebook/jest/pull/5161))
+* `[jest-regex-util]` Fix breaking change in `--testPathPattern`
+  ([#5230](https://github.com/facebook/jest/pull/5230))
 
 ### Features
 

packages/jest-regex-util/src/__tests__/__snapshots__/index.test.js.snap

@@ -0,0 +1,19 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`replacePathSepForRegex() posix should match the expected output from #5216 1`] = `
+Array [
+  "jest-config\\\\/.*normalize",
+  "jest-config/.*normalize",
+  "jest-config\\\\.*normalize",
+  "jest-config\\\\\\\\.*normalize",
+]
+`;
+
+exports[`replacePathSepForRegex() win32 should match the expected output from #5216 1`] = `
+Array [
+  "jest-config\\\\\\\\\\\\\\\\.*normalize",
+  "jest-config\\\\\\\\.*normalize",
+  "jest-config\\\\.*normalize",
+  "jest-config\\\\\\\\.*normalize",
+]
+`;

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

@@ -0,0 +1,69 @@
+jest.mock('path');
+
+import {replacePathSepForRegex} from '../index';
+import path from 'path';
+
+describe('replacePathSepForRegex()', () => {
+  const testPatternsFrom5216 = [
+    'jest-config\\/.*normalize',
+    'jest-config/.*normalize',
+    'jest-config\\.*normalize',
+    'jest-config\\\\.*normalize',
+  ];
+
+  describe('posix', () => {
+    beforeEach(() => (path.sep = '/'));
+
+    it('should return the path', () => {
+      const expected = {};
+      expect(replacePathSepForRegex(expected)).toBe(expected);
+    });
+
+    // Confirming existing behavior; could be changed to improve cross-platform support
+    it('should not replace Windows path separators', () => {
+      expect(replacePathSepForRegex('a\\.*b')).toBe('a\\.*b');
+      expect(replacePathSepForRegex('a\\\\.*b')).toBe('a\\\\.*b');
+    });
+
+    // Bonus: Test cases from https://github.com/facebook/jest#5216
+    it('should match the expected output from #5216', () => {
+      expect(
+        testPatternsFrom5216.map(replacePathSepForRegex),
+      ).toMatchSnapshot();
+    });
+  });
+
+  describe('win32', () => {
+    beforeEach(() => (path.sep = '\\'));
+
+    it('should escape Windows path separators', () => {
+      expect(replacePathSepForRegex('a\\b\\c')).toBe('a\\\\b\\\\c');
+    });
+
+    it('should replace POSIX path separators', () => {
+      expect(replacePathSepForRegex('a/b/c')).toBe('a\\\\b\\\\c');
+    });
+
+    it('should not escape an escaped period', () => {
+      expect(replacePathSepForRegex('a\\.dotfile')).toBe('a\\.dotfile');
+      expect(replacePathSepForRegex('a\\\\\\.dotfile')).toBe('a\\\\\\.dotfile');
+    });
+
+    it('should not escape an escaped Windows path separator', () => {
+      expect(replacePathSepForRegex('a\\\\b')).toBe('a\\\\b');
+      expect(replacePathSepForRegex('a\\\\.dotfile')).toBe('a\\\\.dotfile');
+    });
+
+    // Confirming existing behavior; could be changed to improve cross-platform support
+    it('should not replace escaped POSIX separators', () => {
+      expect(replacePathSepForRegex('a\\/b')).toBe('a\\\\\\\\b');
+    });
+
+    // Bonus: Test cases from https://github.com/facebook/jest#5216
+    it('should match the expected output from #5216', () => {
+      expect(
+        testPatternsFrom5216.map(replacePathSepForRegex),
+      ).toMatchSnapshot();
+    });
+  });
+});

packages/jest-regex-util/src/index.js

@@ -22,8 +22,56 @@ export const escapeStrForRegex = (string: string) =>
   string.replace(/[[\]{}()*+?.\\^$|]/g, '\\$&');
 
 export const replacePathSepForRegex = (string: string) => {
-  if (path.sep === '\\') {
-    return string.replace(/(\/|\\(?!\.))/g, '\\\\');
+  if (!string || path.sep !== '\\') {
+    return string;
+  }
+
+  let result = '';
+  for (let i = 0; i < string.length; i += 1) {
+    const char = string[i];
+    if (char === '\\') {
+      const nextChar = string[i + 1];
+      /* Case: \/ -- recreate legacy behavior */
+      if (nextChar === '/') {
+        i += 1;
+        result += '\\\\\\\\';
+        continue;
+      }
+
+      /* Case: \. */
+      if (nextChar === '.') {
+        i += 1;
+        result += '\\.';
+        continue;
+      }
+
+      /* Case: \\. */
+      if (nextChar === '\\' && string[i + 2] === '.') {
+        i += 2;
+        result += '\\\\.';
+        continue;
+      }
+
+      /* Case: \\ */
+      if (nextChar === '\\') {
+        i += 1;
+        result += '\\\\';
+        continue;
+      }
+
+      /* Case: \<other> */
+      result += '\\\\';
+      continue;
+    }
+
+    /* Case: / */
+    if (char === '/') {
+      result += '\\\\';
+      continue;
+    }
+
+    result += char;
   }
-  return string;
+
+  return result;
 };