Close all HTML unclosed IMG tags

(<img[^>]+)(?<!/)>

will match an img tag that is not properly closed. It requires that the regex flavor you're using supports lookbehind (which Ruby and JavaScript don't but most others do). Backreference no. 1 will contain the match, so if you search for this regex and replace by \1/> you should be good to go.

If you need to account for the possibility of > inside attributes, you could use

(<img("[^"]*"|[^>])+)(?<!/)>

This will match, e.g.,

<img src="image.gif" alt="hey, look--->">
<img src="image/image.gif">

and leave

<img src="image/image.gif" />

alone.

In HTML the end tag for an <img> "must be omitted", so the start tag closes the element and you can't have an unclosed img.

If you want to convert your HTML to XHTML then use a real parser. Regular Expressions aren't a very good tool for this job.