Fix toMatch when re-using RegExp with 'g' flag

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))

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', () => {

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
       ? () =>