Using regular expressions to find img tags without an alt attribute

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.">

This works in Eclipse:

<img(?!.*alt).*?>

I'm updating for Section 508 too!

Building on Mr.Black and Roberts126 answers:

/(<img(?!.*?alt=(['"]).*?\2)[^>]*)(>)/

This will match an img tag anywhere in the code which either has no alt tag or an alt tag which is not followed by ="" or ='' (i.e. invalid alt tags).

Breaking it down:

(          : open capturing group
<img       : match the opening of an img tag
(?!        : open negative look-ahead
.*?        : lazy some or none to match any character
alt=(['"]) : match an 'alt' attribute followed by ' or " (and remember which for later)
.*?        : lazy some or none to match the value of the 'alt' attribute
\2)        : back-reference to the ' or " matched earlier
[^>]*      : match anything following the alt tag up to the closing '>' of the img tag
)          : close capturing group
(>)        : match the closing '>' of the img tag

If your code editor allows search and replace by Regex you can use this in combination with the replace string:

$1 alt=""$3

To find any alt-less img tags and append them with an empty alt tag. This is useful when using spacers or other layout images for HTML emails and the like.