Kevin Powell - Community•10mo ago

RegExp with g flag with weird behaviour. why?

i was debugging a bug and found this incredibly weird behaviour with a regex. the following code was returning false on things that obviously match:

var regex = new RegExp('a', 'g');
console.log(regex.test('a')); // true
console.log(regex.test('a')); // false

var regex = new RegExp('a', 'g');
console.log(regex.test('a')); // true
console.log(regex.test('a')); // false

after trial and error, i found that re-using the same object with the g flag caused the behaviour above. removing the g flag makes it work properly. (using a new literal regex object fixes it too, but what i was debugging can't be made into a literal regex, since it is dynamically generated.) is there any reason for this behaviour? mdn doesnt have anything obvious that could clue that this behaviour could happen.

3 Replies

ChooKing•10mo ago

Did you read MDN? It explains how the behavior is different when using the global flag. If you do it a third time, you will get true again, because it resets. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/test

RegExp.prototype.test() - JavaScript | MDN

The test() method of RegExp instances executes a search with this regular expression for a match between a regular expression and a specified string. Returns true if there is a match; false otherwise.

ἔρως•10mo ago

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/global <-- i read this

RegExp.prototype.global - JavaScript | MDN

The global accessor property of RegExp instances returns whether or not the g flag is used with this regular expression.

ἔρως•10mo ago

not that but that explains the idiotic behaviour

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RegExp with g flag with weird behaviour. why?