JS RegExp在FireBug中匹配，但不是JSFiddle？

Question

I'm pretty stumped as to what's wrong with the following regular expression:

var // this regex digests a string into leading whitespace,
    // content text, and trailing whitespace.
    lineRegex = /^(\s*)(\S.*\S|\S)?(\s*)$/g,
    /* ... */;

lineRegex is supposed to match anything . There is not even one required character, and if there are characters in the string, they are all either white space or not white space. When I paste the exact regex into FireBug console and add .exec("") , I get the expected result, ["", "", undefined, ""] . But in my JSFiddle ( http://jsfiddle.net/Lay9k/9/ ) I have:

try {
    var sections = lineRegex.exec(lineTxt),
        rawLeadingWs = sections[1] || "",
        contentText = sections[2] || "",
        importantTabs = rawLeadingWs.replace(
            oneTabWorthOfSpaces, "\t"
        ),
        tabsAndAligningSpaces =
            aligningTabRegex.exec(importantTabs),
        // I don't like this as much as Lint does.
        importantWsLength = 
            (tabsAndAligningSpaces[1] || "").replace(/[^\t]/g, "")
            .length * spacesPerTab +
            (tabsAndAligningSpaces[2] || "").length;
} catch (wtf) {
    console.log({
        lineText: lineTxt,
        sections: sections,
        rawLeadingWs: rawLeadingWs,
        contentText: contentText,
        importantTabs: importantTabs,
        tabsAndAligningSpaces: tabsAndAligningSpaces,
        importantWsLength: importantWsLength
    });
    throw wtf;
}

and I get sections is null when lineTxt is "" . So what gives?

Answer 1

I think the problem is that the lastIndex property of the regex is not reset properly.

Here's a related issue: Why does Javascript's regex.exec() not always return the same value?

reg.lastIndex = 0; from one of the answers seems to fix it.