一个小正则问题的研究

var reg =/123/g;
var str = "123#abc";
console.log(reg.test(str)); //true
console.log(reg.test(str)); //false
console.log(reg.test(str)); //true
console.log(reg.test(str)); //false

1. Let S be the value of ToString(string).
2. Let length be the length of S.
3. Let lastIndex be the value of the lastIndex property.
4. Let i be the value of ToInteger(lastIndex).
5. If the global property is false, let i = 0.
6. If I < 0 or I > length then set lastIndex to 0 and return null.
7. Call [Match], giving it the arguments S and i. If [Match] returned failure, go to step 8;
otherwise let r be its State result and go to step 10.
8. Let i = i+1.
9. Go to step 6.
10. Let e be r's endIndex value.- 145 -
11. If the global property is true, set lastIndex to e.
12. Let n be the length of r's captures array. (This is the same value as 15.10.2.1's
NCapturingParens.)
13. Return a new array with the following properties:
• The index property is set to the position of the matched substring within the complete string
S.
• The input property is set to S.
• The length property is set to n + 1.
• The 0 property is set to the matched substring (i.e. the portion of S between offset i inclusive
and offset e exclusive).
• For each integer i such that I > 0 and I ≤ n, set the property named ToString

to the i
th element
of r's captures array.

var reg =/123/g;
var str = "123#abc";
console.log(reg.lastIndex); //0
console.log(reg.test(str)); //true
console.log(reg.lastIndex); //3
console.log(reg.test(str)); //false
console.log(reg.lastIndex); //0
console.log(reg.test(str)); //true
console.log(reg.lastIndex); //3
console.log(reg.test(str)); //false
console.log(reg.lastIndex); //0