Why Does RegExp's `test()` Method Return Unexpected Results with Global and Case-Insensitive Flags?

RegExp with Global Flag and Case Insensitive Flag Yields Unexpected Results

Question:

When using a regular expression with the global flag (g) and the case insensitive flag (i), why does the test method produce incorrect results for user-generated input?

Example:

Consider the following code:

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]

Expected result: [true, true]

Explanation:

A RegExp object with the g flag maintains the lastIndex property, indicating the position of the last match. When the test method is called repeatedly without resetting lastIndex, it will resume the search from the last used index rather than starting from 0.

Demonstration:

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

console.log(re.test('Foo Bar')); // true
console.log(re.lastIndex); // 6

console.log(re.test('Foo Bar')); // false
console.log(re.lastIndex); // 6

In the above example, the first call to test sets lastIndex to 6. Subsequently, the second call resumes the search from index 6, which results in false as the search scope is limited to the characters following the last match.

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