Replacing partial regex matches in place with Ruby

A quick solution (adjust as necessary):

s = 'This is a ![foto](foto.jpeg)'

s.sub!(/!(\[.*?\])\((.*?)\)/, '\1(/folder1/\2)' )

p s  # This is a [foto](/folder1/foto.jpeg)

You can always do it in two steps - first extract the whole image expression out and then second replace the link:

str = "This is a ![foto](foto.jpeg), here is another ![foto](foto.png)"

str.gsub(/\!\[[^\]]*\]\(([^)]*)\)/) do |image|
  image.gsub(/(?<=\()(.*)(?=\))/) do |link|
    "/a/new/path/" + link
  end
end

#=> "This is a ![foto](/a/new/path/foto.jpeg), here is another ![foto](/a/new/path/foto.png)"

I changed the first regex a bit, but you can use the same one you had before in its place. image is the image expression like ![foto](foto.jpeg), and link is just the path like foto.jpeg.

[EDIT] Clarification: Ruby does have lookbehinds (and they are used in my answer):

You can create lookbehinds with (?<=regex) for positive and (?<!regex) for negative, where regex is an arbitrary regex expression subject to the following condition. Regexp expressions in lookbehinds they have to be fixed width due to limitations on the regex implementation, which means that they can't include expressions with an unknown number of repetitions or alternations with different-width choices. If you try to do that, you'll get an error. (The restriction doesn't apply to lookaheads though).

In your case, the [foto] part has a variable width (foto can be any string) so it can't go into a lookbehind due to the above. However, lookbehind is exactly what we need since it's a zero-width match, and we take advantage of that in the second regex which only needs to worry about (fixed-length) compulsory open parentheses.

Obviously you can put real_path in from here, but I just wanted a test-able example.

I think that this approach is more flexible and more readable than reconstructing the string through the match group variables

In your block, use $1 to access the first capture group ($2 for the second and so on).

From the documentation:

In the block form, the current match string is passed in as a parameter, and variables such as $1, $2, $`, $&, and $' will be set appropriately. The value returned by the block will be substituted for the match on each call.