Why Does a Global Flag in RegExp Produce Unexpected Results with Multiple Tests?

Why the Global Flag can Produce Unexpected RegExp Results

Regular expressions (RegExps) with the global flag (g) are typically used to find multiple occurrences of a pattern within a string. However, using this flag together with the case-insensitive flag (i) may lead to unexpected results.

Consider the following example:

var query = 'Foo B';
var re = new RegExp(query, 'gi');
var result = [];
result.push(re.test('Foo Bar'));
result.push(re.test('Foo Bar'));
// result will be [true, false]

In this scenario, the test method only returns true in the first iteration. To understand this behavior, we need to consider how the g flag works.

Last Index Tracking with the g Flag

When used with the g flag, a RegExp object keeps track of the last index where a match occurred. On subsequent matches, the RegExp will start searching from this last index instead of 0.

Illustrating this concept:

var query = 'Foo B';
var re = new RegExp(query, 'gi');
console.log(re.lastIndex); // Initial value is 0

console.log(re.test('Foo Bar')); // Matches at index 0
console.log(re.lastIndex); // Updated to 6

console.log(re.test('Foo Bar')); // No match, lastIndex is still 6
console.log(re.lastIndex); // Remains at 6

In this example, even though the Foo Bar string contains another match, the second test call returns false because the RegExp has already moved its search position past the match.

Therefore, using the g flag in conjunction with the i flag can result in missed matches if the search string contains multiple instances of the pattern with varying case. To avoid this, consider using the i flag without the g flag or using a single test call with the g flag to find the first match only.

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