match() returns array with two matches when I expect one match
Consider the following example:
<html>
<body>
<script type="text/javascript">
var str="filename.jpg";
var pattOne = new RegExp('\.[^\.]*$');
var pattTwo = new RegExp('(\.[^\.]*$)');
var pattThree = new RegExp('(\.[^\.]*$)', 'g');
document.write(str.match(pattOne));
document.write('<br>');
document.write(str.match(pattTwo));
document.write('<br>');
document.write(str.match(pattThree));
</script>
</body>
</html>
Here is the result:
.jpg
.jpg,.jpg
.jpg
I expect this result:
.jpg
.jpg
.jpg
Why placing parenthesis around the regular expression changes the result? Why using 'g' modifier changes again the result?
Answer
From String.prototype.match [MDN]:
If the regular expression does not include the
gflag, returns the same result asregexp.exec(string).
Where the RegExp.prototype.exec documentation [MDN] says:
The returned array has the matched text as the first item, and then one item for each capturing parenthesis that matched containing the text that was captured.
So as you introduced a capture group in the second expression, the first element is the whole match and the second contains the content of the capture group, which, in your example, is the whole match as well.
The first expression does not have a capture group, so you only get back the match.
Back to the match documentation:
If the regular expression includes the
gflag, the method returns an Array containing all matches. If there were no matches, the method returnsnull.
With the g modifier, only matches are returned, but not the content of capture groups. In your string there is only one match.