From dadc9b89d9f18be830777bed0e2cdacffb11bf93 Mon Sep 17 00:00:00 2001
From: Robert Fletcher <lobatifricha@gmail.com>
Date: Mon, 9 Dec 2019 20:33:17 -0800
Subject: [PATCH] Fix `toMatch` when re-using RegExp with 'g' flag

Fixes #9283

When testing a regex with the `g` flag, the regular expression is not
reset between assertions, such that subsequent assertions that should
pass, fail. The test below passes on Jest 23 but fails on 24:

```ts
it('matches consistently when re-using a regex with the "g" flag', () => {
  const regex = /foo/g;
  jestExpect('foo').toMatch(regex); // passes
  jestExpect('foo').toMatch(regex); // fails!
});
```

It appears this behavior was introduced in c3a0167. Previously, Jest
cloned the regex with `new RegExp()`, which sidestepped the issue by
using a new regex for every assertion. This restores that behavior. The
issue here is due to somewhat confusing behavior in the way that regular
expressions with the `g` or `y` flag behave. Internally, the regex
[keeps a `lastIndex`][1] and the next time it is matched against, it
will search from that index, or cycle back to 0 if no match is found:

```
> regex = /foo/g
/foo/g
> regex.test('foo')
true
> regex.lastIndex
3
> regex.test('foo')
false
> regex.lastIndex
0
> regex.test('foo')
true
> regex.lastIndex
3
```

[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/lastIndex
---
 CHANGELOG.md                                   | 1 +
 packages/expect/src/__tests__/matchers.test.js | 6 ++++++
 packages/expect/src/matchers.ts                | 2 +-
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4160a719cd8d..de90052578d6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -46,6 +46,7 @@
 - `[expect]` Avoid incorrect difference for subset when `toMatchObject` fails ([#9005](https://github.com/facebook/jest/pull/9005))
 - `[expect]` Consider all RegExp flags for equality ([#9167](https://github.com/facebook/jest/pull/9167))
 - `[expect]` [**BREAKING**] Consider primitives different from wrappers instantiated with `new` ([#9167](https://github.com/facebook/jest/pull/9167))
+- `[expect]` Fix `toMatch` to return consistent results when re-using `RegExp` with `g` flag ([#9290](https://github.com/facebook/jest/pull/9290))
 - `[jest-config]` Use half of the available cores when `watchAll` mode is enabled ([#9117](https://github.com/facebook/jest/pull/9117))
 - `[jest-config]` Fix Jest multi project runner still cannot handle exactly one project ([#8894](https://github.com/facebook/jest/pull/8894))
 - `[jest-console]` Add missing `console.group` calls to `NullConsole` ([#9024](https://github.com/facebook/jest/pull/9024))
diff --git a/packages/expect/src/__tests__/matchers.test.js b/packages/expect/src/__tests__/matchers.test.js
index fb6223a9466f..f98cb99224c8 100644
--- a/packages/expect/src/__tests__/matchers.test.js
+++ b/packages/expect/src/__tests__/matchers.test.js
@@ -1617,6 +1617,12 @@ describe('.toMatch()', () => {
   it('escapes strings properly', () => {
     jestExpect('this?: throws').toMatch('this?: throws');
   });
+
+  it('matches consistently when re-using a regex with the "g" flag', () => {
+    const regex = /foo/g;
+    jestExpect('foo').toMatch(regex);
+    jestExpect('foo').toMatch(regex);
+  });
 });
 
 describe('.toHaveLength', () => {
diff --git a/packages/expect/src/matchers.ts b/packages/expect/src/matchers.ts
index ca51e52291ba..58c015992aeb 100644
--- a/packages/expect/src/matchers.ts
+++ b/packages/expect/src/matchers.ts
@@ -835,7 +835,7 @@ const matchers: MatchersObject = {
     const pass =
       typeof expected === 'string'
         ? received.includes(expected)
-        : expected.test(received);
+        : new RegExp(expected).test(received);
 
     const message = pass
       ? () =>