Question

Possible Duplicate:
Javascript regex returning true.. then false.. then true.. etc

First of all, apologize for my bad english.

I’m trying to test string to match the pattern, so I has wrote this:

var str = 'test';
var pattern = new RegExp('te', 'gi'); // yes, I know that simple 'i' will be good for this

But I have this unexpected results:

>>> pattern.test(str)
true
>>> pattern.test(str)
false
>>> pattern.test(str)
true

Can anyone explain this?

Answer 1

The reason for this behavior is that RegEx isn’t stateless. Your second test will continue to look for the next match in the string, and reports that it doesn’t find any more. Further searches starts from the beginning, as lastIndex is reset when no match is found:

var pattern = /te/gi;

pattern.test('test');
>> true
pattern.lastIndex;
>> 2

pattern.test('test');
>> false
pattern.lastIndex;
>> 0

You’ll notice how this changes when there are two matches, for instance:

var pattern = /t/gi;

pattern.test('test');
>> true
pattern.lastIndex;
>> 1

pattern.test('test');
>> true
pattern.lastIndex;
>> 4

pattern.test('test');
>> false
pattern.lastIndex;
>> 0