How to return the substring of a string between two strings in Ruby?

Just split it twice and get the string between the markers:

input_string.split("str1_markerstring").last.split("str2_markerstring").first

input_string = "blahblahblahSTARTfoofoofooENDwowowowowo"
str1_markerstring = "START"
str2_markerstring = "END"

input_string[/#{str1_markerstring}(.*?)#{str2_markerstring}/m, 1]
#=> "foofoofoo"

or to put it in a method:

class String
  def string_between_markers marker1, marker2
    self[/#{Regexp.escape(marker1)}(.*?)#{Regexp.escape(marker2)}/m, 1]
  end
end

"blahblahblahSTARTfoofoofooENDwowowowowo".string_between_markers("START", "END")
#=> "foofoofoo"