用regexp替换返回undefined

Question

I'm not so bad in regular expression but there I have a tiny problem, maybe one of you can answer this simple question.

I do this:

var myString = "10px";
var myRegexp = new RegExp("(\\d*\\.?\\d*){1}(em|ex|grad|ch|deg|ms|rad|rem|s|turn|vh|vw|vmin|vmax|px|cm|in|pt|pc|%)?", "gi");

myString.replace(myRegexp, function( match, number, type ){
    console.log(match, number, type);
});

The replace console log two time, the first is : "10px", "10", "px" and I don't know why there is a second time, it return me that: ,,undefined (the two first value are not null or empty string, they return just nothing, absolutely empty).

My question, why the replace try a seconde replacing on nothing ? And how avoid it ?

Answer 1

Instead of:

 (\\d*\\.?\\d*){1}

Use start anchor:

^(\\d*\\.?\\d*)

As your regex has everything optional and matches empty string also.

Answer 2

I think it is because your expression has everything optional, so you are also matching a zero-length string (an issue with javascript). If I understand what you want to do, I don't think the number has to be optional, so you could try:

(\d*\.?\d+)(em|ex|grad|ch|deg|ms|rad|rem|s|turn|vh|vw|vmin|vmax|px|cm|in|pt|pc|%)?