Regular Expressions Cookbook

8.8. Remove XML-Style Comments

Problem

You want to remove comments from an (X)HTML or XML document. For example, you want to remove comments from a web page before it is served to web browsers in order to reduce the file size of the page and thereby reduce load time for people with slow Internet connections.

Solution

Finding comments is not a difficult task, thanks to the availability of lazy quantifiers. Here is the regular expression for the job:

<!--.*?-->

Regex options: Dot matches line breaks

Regex flavors: .NET, Java, PCRE, Perl, Python, Ruby

That’s pretty straightforward. As usual, though, JavaScript’s lack of a “dot matches line breaks” option means that you’ll need to replace the dot with an all-inclusive character class in order for the regular expression to match comments that span more than one line. Following is a version that works with JavaScript:

<!--[\s\S]*?-->

Regex options: None

Regex flavor: JavaScript

To remove the comments, replace all matches with the empty string (i.e., nothing). Recipe 3.14 lists code to replace all matches of a regex.

Discussion

How it works

At the beginning and end of this regular expression are the literal character sequences ‹<!--› and ‹-->›. Since none of those characters are special in regex syntax (except within character classes, where hyphens create ranges), they don’t need to be escaped. That just leaves the ‹.*?› or ‹[\s\S]*?› in the middle of the regex to examine further.

Thanks to the “dot matches line breaks” option, the dot in the regex ...