Question

I was just reviewing a previous post I made and noticed a number of people suggesting that I don’t use Regex to parse xml. In that case the xml was relatively simple, and Regex didn’t pose any problems. I was also parsing a number of other code formats, so for the sake of uniformity it made sense. But I’m curious how this might pose a problem in other cases. Is this just a ‘don’t reinvent the wheel’ type of issue?

Answer 1

The real trouble is nested tags. Nested tags are very difficult to handle with regular expressions. It’s possible with balanced matching, but that’s only available in .NET and maybe a couple other flavors. But even with the power of balanced matching, an ill-placed comment could potentially throw off the regular expression.

For example, this is a tricky one to parse…

<div>
    <div id="parse-this">
        <!-- oops</div> -->
        try to get this value with regex
    </div>
</div>

You could be chasing edge cases like this for hours with a regular expression, and maybe find a solution. But really, there’s no point when there are specialized XML, XHTML, and HTML parsers out there that do the job more reliably and efficiently.