Fix toMatch when re-using RegExp with 'g' flag
#9290
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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
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Ah, woops. Just saw that @wsmd beat me to the punch. |
Codecov Report
@@ Coverage Diff @@
## master #9290 +/- ##
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- Coverage 64.98% 64.97% -0.02%
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Files 278 278
Lines 11899 11899
Branches 2934 2933 -1
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- Hits 7733 7731 -2
- Misses 3537 3538 +1
- Partials 629 630 +1
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Summary
Fixes #9283
When testing a regex with the
gflag, the regular expression is notreset between assertions, such that subsequent assertions that should
pass, fail. The test below passes on Jest 23 but fails on 24:
It appears this behavior was introduced in c3a0167. Previously, Jest
cloned the regex with
new RegExp(), which sidestepped the issue byusing 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
goryflag behave. Internally, the regexkeeps a
lastIndexand the next time it is matched against, itwill search from that index, or cycle back to 0 if no match is found:
Test plan
Added a new test.