Finding img src attributes

I’m having trouble turning up img src attributes with this plugin. I need to update all references to src=”http://subdomain.blog.org/files/ with src=”http://domain.org/files

When I run a basic ‘post content’ search with regex for http://subdomain.blog.org/files/ it turns up zero results even though I know there are several in the bodies of my posts.

Am I doing something wrong or does the plugin not search img src attributes?

https://wordpress.org/plugins/search-regex/