Using regular expressions to find img tags without an alt attribute

Question

I am going through a large website (1600+ pages) to make it pass Priority 1 W3C WAI. As a result, things like image tags need to have alt attributes.

What would be the

Answer 1

This is perfectly possible with following regEx:

<img([^a]|a[^l]|al[^t]|alt[^=])*?/>

Looking for something that isn't there, is rather tricky, but we can trick them back, by looking for a group that doesn't start with 'a', or an 'a' that doesn't get followed by an 'l' and so on.

Answer 2

Here is what I just tried in my own environment with a massive enterprise code base with some good success (found no false positives but definitely found valid cases):

<img(?![^>]*\balt=)[^>]*?>

What's going on in this search:

1. find the opening of the tag
2. look for the absence of zero or more characters that are not the closing bracket while also …
3. Checking for the absence of of a word that begins with "alt" ("\b" is there for making sure we don't get a mid-word name match on something like a class value) and is followed by "=", then …
4. look for zero or more characters that are not the closing bracket
5. find the closing bracket

So this will match:

<img src="foo.jpg" class="baltic" />

But it won't match either of these:

<img src="foo.jpg" class="baltic" alt="" />
<img src="foo.jpg" alt="I have a value.">