为什么此正则表达式在javascript中匹配？

Question

Fiddle at http://jsfiddle.net/42zcL/

I have the following code, which should alert "No Match". If I put the regex into regexpal.com and run it, it doesn't match (as expected). With this code, it does match. I know there is another way to do it, which works correctly - /^((.*)Waiting(.*))?$/ , but I am curious as to why this one fails. It should match a string with the text "Waiting" in it or nothing at all.

var teststring="Anything";
if (teststring.match(/^((.*)Waiting(.*))|()$/)) alert('match');
else alert('No Match');

---

EDIT: Clearer example:

var teststring="b";
if (teststring.match(/^(a)|()$/)) alert('match');
else alert('No Match');

Produces a Match, when I would expect "No Match"

Expected behaviour, as per regexpal.com:

teststring: a = match
teststring: b = no match

Actual behaviour in javascript:

teststring: a = match
teststring: b = match

Answer 1

Because you have |()$ at the end which is like saying "Match what comes before | but if you don't find it, match anything as long as there's an end of line."

- Full RegEx reference

- Try it out

Hopefully this explains it a little better:

1. The use of () in RegEx does not mean " Don't match anything ". If no characters are specified it will still match against () at each position in the string (letter position that is). Imagine it like this: The word "Anything" turned into an array - [A,n,y,t,h,i,n,g] - if n = length of that array , the placeholder at [n] is non-empty , resulting in a "match" since no specific restriction was expressed in the pattern.
2. Since #1 essentially means |()$ will return a positive result on any word tested , you will always see "match" in your alert.

I'm pretty terrible at conveying my thoughts so maybe this previous stack answer will fill in whatever holes my answer left open.