Using the Regex Global Flag with Test

June 26, 2019

3 min read

Using regexes in JavaScript can be tricky! One new gotcha I ran into today involves the usage of the g or global flag. The global flag matches against all occurences of the regex. This can be useful to find how many times a regex is matched inside a phrase.

You can enable the g flag in a few ways:

const regexSyntax = /test/g;
const regexConstructed = new RegExp('test', 'g');

When you use the g flag in combination with the regex test function you can run into some unexpected behavior.

const regex = /phrase/g;

// command:           // output:
regex.test('phrase'); // true
regex.test('phrase'); // false
regex.test('phrase'); // true
regex.test('phrase'); // false

The above occurs because the regex object will save the lastIndex of a match between calls when it has the g property enabled. The next time test is called, the regex will start matching at this lastIndex value. Once test is called and no match is found after the current lastIndex value, it is reset to 0.

Here is the above code snippet wth the addition of logging lastIndex before and after every test call.

const regex = /phrase/g;

// command:           // output:
regex.lastIndex       // 0
regex.test('phrase'); // true
regex.lastIndex       // 6
regex.test('phrase'); // false
regex.lastIndex       // 0
regex.test('phrase'); // true
regex.lastIndex       // 6
regex.test('phrase'); // false
regex.lastIndex       // 0

This happens because a global regex test is intended to be called multiple times in a row on the same input.

Here's an example where we expect to get multiple matches on an input string:

const regex = /phrase/g;

// command:                         // output:
regex.lastIndex                     // 0
regex.test('phrase phrase phrase'); // true
regex.lastIndex                     // 6
regex.test('phrase phrase phrase'); // true
regex.lastIndex                     // 13
regex.test('phrase phrase phrase'); // true
regex.lastIndex                     // 20
regex.test('phrase phrase phrase'); // false
regex.lastIndex                     // 0

#sources

https://stackoverflow.com/questions/2851308/why-does-my-javascript-regex-test-give-alternating-results https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/global