Allowed snippet:
allowed = function(url, parenturl)
  local blog = read_file("blog")
  local concat = "https?://9volt-art.tumblr.com/"
  print("Before reject")
  if string.match(url, "'+")
  or string.match(url, "[<>\\%*%$;%^%[%],%(%)]")
  or string.match(url, "//$") then
    print("After rejection")
    return false
  end
  print("Before acceptance")
  if string.gmatch(url, concat) then
    print("After acceptance")
    return true
  end

  return false
end

Output:
128=200 http://assets.tumblr.com/fonts/fairwater/fairwater_script_regular-webfont.woff?v=96e975b7468359e6f67086305577e43e
Before reject
Before acceptance
After acceptance
Before reject
After rejection
129=200 http://assets.tumblr.com/fonts/sourcecodepro/sourcecodepro-regular-webfont.woff2?v=c2bb363563f7a15a4c0fe91cf951a67c
Before reject
Before acceptance
After acceptance
Before reject
After rejection
Before reject
After rejection
Before reject
Before acceptance
After acceptance
Before reject
After rejection
130=200 http://assets.tumblr.com/fonts/sourcecodepro/sourcecodepro-regular-webfont.woff?v=c5fa36698a378c9c64711cec95efd69e
Before reject
Before acceptance
After acceptance
Before reject
After rejection
Segmentation fault (core dumped)