%r{}
is equivalent to the /.../
notation, but allows you to have “https://stackoverflow.com/” in your regexp without having to escape them:
%r{/home/user}
is equivalent to:
/\/home\/user/
This is only a syntax commodity, for legibility.
Edit:
Note that you can use almost any non-alphabetic character pair instead of ‘{}’.
These variants work just as well:
%r!/home/user!
%r'/home/user'
%r(/home/user)
Edit 2:
Note that the %r{}x
variant ignores whitespace, making complex regexps more readable. Example from GitHub’s Ruby style guide:
regexp = %r{
start # some text
\s # white space char
(group) # first group
(?:alt1|alt2) # some alternation
end
}x