regex.js

var r = new RegExp(
  //   ^  - not the following...
  //   \/\? - slash and a question mark
  //   +  - 1 or more of them
  '[^/?]+' 

  // Followed by (but don't capture this!)...
  //   ?= - don't capture!
  //   ^  - not the following...
  //   /  - a slash
  //   *  - 0 or more of those
  //   $  - followed by the end of line
  + '(?=[^/]+$)'
);

console.log('http://a.com/b/c/this_is_what_we_want?q=poop'.match(r).toString());