Regular expression with javascript

Question

I have following code in java script

    var regexp = /\$[A-Z]+[0-9]+/g;
    for (var i = 0; i < 6; i++) {
        if (regexp.test("$A1")) {
            console.log("Matched");
        }
        else {
            console.log("Unmatched");
        }
    }

Please run it on your browser console. It will print alternative Matched and Unmatched. Can anyone tell the reason for it.

Answer 1

After call test on a string, the lastIndex pointer will be set after the match.

Before:
$A1
^

After:
$A1
   ^

and when it comes to the end, the pointer will be reset to the start of the string.

You can try '$A1$A1', the result will be

Matched
Matched
Unmatched
...

This behavior is defined in 15.10.6.2, ECMAScript Language Spec.

Step 11. If global is true, a. Call the [[Put]] internal method of R with arguments "lastIndex", e, and true.

Answer 2

I've narrowed your code down to a simple example:

var re = /a/g, // global expression to test for the occurrence of 'a'
s = 'aa';     // a string with multiple 'a'

> re.test(s)
  true
> re.lastIndex
  1
> re.test(s)
  true
> re.lastIndex
  2
> re.test(s)
  false
> re.lastIndex
  0

This only happens with global regular expressions!

From the MDN documentation on .test():

As with exec (or in combination with it), test called multiple times on the same global regular expression instance will advance past the previous match.