Why do regular expressions for global flags cause incorrect output?

Question

What is wrong with this regex when I use global flags and case insensitive flags? Queries are user-generated input. The result should be [true, true].

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

---

<code>var reg = /^a$/g;
for(i = 0; i < 10;)
   console.log(reg.test("a"));</code>

Answer 1

You are using a RegExp object and executing it multiple times. On each execution, it continues from the last matching index.

Before each execution, you need to "reset" the regex to start from scratch:

result.push(re.test('Foo Bar'));
re.lastIndex = 0;
result.push(re.test('Foo Bar'));
// 现在的结果是 [true, true]

Having said that, it might be more readable to create a new RegExp object each time (the overhead is minimal since the RegExp is already cached):

result.push((/Foo B/gi).test(stringA));
result.push((/Foo B/gi).test(stringB));

Answer 2

Using a RegExp object with the g flag will keep track of the lastIndex where the match occurred, so on subsequent matches it will Start from the last used index, not from 0. Take a look at the example:

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

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

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