Markdown -> Showdown bug in detab regex?

Question

I\'m looking at Gruber\'s original Markdown implementation here and the Showdown implementation here.

I\'m comparing the _Detab function in each. I\'m

Answer 1

It looks like a bug in the Showdown implementation. Markdown uses 4-space tabs, so a string ending in a tab should always be a multiple of 4 characters long after tabs are converted to spaces. The Perl version makes "Where\t" 8 characters, but the JavaScript one makes it 9 characters.

I suspect the bug may not occur with tabs at the beginning of a line, which is how they're normally used in Markdown, which would explain why it hasn't been noticed.

Answer 2

There are several bugs in Showdown's detabber. That's why for Stack Overflow's version, I have rewritten it:

function _Detab(text) {
    if (!/\t/.test(text))
        return text;

    var spaces = ["    ", "   ", "  ", " "],
    skew = 0,
    v;

    return text.replace(/[\n\t]/g, function (match, offset) {
        if (match === "\n") {
            skew = offset + 1;
            return match;
        }
        v = (offset - skew) % 4;
        skew = offset + 1;
        return spaces[v];
    });
}

It detabs correctly, and if I recall my measurements correctly, this is about as fast (maybe a little slower) as the original in older IE versions, and much faster in newer browsers.

See http://code.google.com/p/pagedown/wiki/PageDown for our full version of Showdown.