尝试使用preg_match_all匹配包含特定类的所有图像

Question

I try to match all images that contain a specific class by using the preg_match_all function.

This is my old code that match the all images in the content:

preg_match_all( '/<img[\s
]+.*?>/is', $content, $matches );

And this is the code that I try to made to match the images that only have a specific class but I failed:

preg_match_all( "/(<img((?!(.*?)class=['\"](.*?)comment-media(.*?)['\"](.*?)).)*>)+/is", $content, $matches );

The class that I need to search for it is: comment-media

Answer 1

Your regex matches every img tag that has not the comment-media class. Regex101 lists this part as being "Negative Lookahead":

(?!(.*?)class=['\"](.*?)comment-media(.*?)['\"](.*?))

You'll need to remove that negative lookahead.

The resulting regex expression will be:

(<img(.*class=['\"](.*)comment-media(.*)['\"].*)*>)+

See it in action here: https://regex101.com/r/yh7dqj/1

---

What is a negative lookahead?

A negative lookahead is used to match something not followed by something else. It always starts with (?!. For example, to match a string foo not followed by bar, one would use:

foo(?!bar)

To match foobar not followed by digits, one would use:

# Notice the parenthesis around the part of the negative lookahead. You'll need these to use lookaheads with regex expressions.
foobar(?!([0-9]+)).*

Note that the lookahead is not part of a capturing group.

More details are available here.