Stripping HTML Comments With PHP But Leaving Conditionals

Question

I\'m currently using PHP and a regular expression to strip out all HTML comments from a page. The script works well... a little too well. It strips out all comments includin

Answer 1

Since comments cannot be nested in HTML, a regex can do the job, in theory. Still, using some kind of parser would be the better choice, especially if your input is not guaranteed to be well-formed.

Here is my attempt at it. To match only normal comments, this would work. It has become quite a monster, sorry for that. I have tested it quite extensively, it seems to do it well, but I give no warranty.

).)*-->

Explanation:

)           #11:   a position not followed by "-->"
  .                 #12:   eat the following char, it's part of the comment
)*                  #13: end non-capturing group, repeat
-->                 #14: "-->"

Steps #02 and #11 are crucial. #02 makes sure that the following characters do not indicate a conditional comment. After that, #11 makes sure that the following characters do not indicate the end of the comment, while #12 and #13 cause the actual matching.

Apply with "global" and "dotall" flags.

To do the opposite (match only conditional comments), it would be something like this:

).)*

Explanation:

  #06:     "" or "" (depends on #02)
  )                 #07:   end of look-ahead
  .                 #08:   eat the following char, it's part of the comment
)*                  #09: end of non-capturing group, repeat
      #10: "" or "" (depends on #02)

Again, apply with "global" and "dotall" flags.

Step #02 is because of the "downlevel-revealed" syntax, see: "MSDN - About Conditional Comments".

I'm not entirely sure where spaces are allowed or expected. Add \s* to the expression where appropriate.