Example 'find what' regex: url\(['"]*(?<url>[^<%|^'<%|^"<%].*?[^'"])['"]*\)
Example 'replace with' regex: url("<%= asset_path '$+{url}' %>")
I used this regex to find url('path/my_image_file.png') and convert it to url("<%= asset_path 'path/my_image_file.png' %>").
Tips:
(?<named>expression) is a named subexpression, and it can be accessed on the 'replace with' field using: $+{named}.
Sublime uses the Perl REGEX engine.