如何使用PHP正则表达式将属性添加到第一个P标记？

Question

WordPress spits posts in this format:

<h2>Some header</h>
<p>First paragraph of the post</p>
<p>Second paragraph of the post</p>
etc.

To get my cool styling on the first paragraph (it's one of those things that looks good only sparingly) I need to hook into the get_posts function to filter its output with a preg_replace.

The goal is to get the above code to look like:

<h2>Some header</h>
<p class="first">First paragraph of the post</p>
<p>Second paragraph of the post</p>

I have this so far but it's not even working (the error is: "preg_replace() [function.preg-replace]: Unknown modifier ']'")

$output=preg_replace('<p[^>]*>', '<p class="first">', $content);

I can't use CSS3 meta-selectors because I need to support IE6, and I can't apply the :first-line meta-selector (this is one that IE6 supports) on the parent container because it would hit the H2 instead of the first P.

Answer 1

Reading through the answers there are some that will work but all have drawbacks of either using an external parsing library or possibly matching tags other than the P tag or also matching its attributes.

I ended up using this solution with the str_replace_once function from here:

str_replace_once('<p>', '<p class="first">', $content);

Simple enough and it works just as intended. Here's full WordPress code snippet to filter the first paragraph any time the_content() is called:

add_filter('the_content', 'first_p_style');
function first_p_style($content) {
 $output=str_replace_once('<p>', '<p class="first">', $content);
 return ($output);
}

Thanks for all the answers!