How to use regex in a Capybara finder?

You can check that an attribute starts with a certain value using ^=:

find("img[src^='https://www.example.com/image']")

The article, The Skinny on CSS Attribute Selectors, describes the various checks (equals, starts with, ends with, etc.):

#Equals
find("img[src='https://www.example.com/image']")

#Contains somewhere
find("img[src*='https://www.example.com/image']")

#Begins with
find("img[src^='https://www.example.com/image']")

#Ends with
find("img[src$='https://www.example.com/image']")