Why Does `RegExp` with Global and Case-Insensitive Flags Return Unexpected Results?

Global RegExp Confusion

The global flag (g) in a regular expression is designed to search for all occurrences of the pattern in a string, as opposed to just the first one. However, when used with the case insensitive flag (i), it might not yield the expected results.

The Problem

Consider the following scenario:

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]

The Reason

The problem arises because the global flag causes RegExp objects to track the last index where a match occurred. In subsequent matches, the search starts from this index instead of 0. This is what happens in the example above:

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

As you can see, the second match resulted in "false" because the search started from index 6, where the match already occurred during the first test.

Resolving the Issue

To avoid this issue, you can reset the RegExp object's lastIndex property between matches:

var query = 'Foo B';
var re = new RegExp(query, 'gi');
var result = [];
result.push(re.test('Foo Bar'));
re.lastIndex = 0;  // Reset the last index
result.push(re.test('Foo Bar'));
// result will be [true, true]

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