function checkisbn( frame ) 	local isbn_str = frame.args[1] 	if nil ~= isbn_str:match("[^%s-0-9X]") then 		return false 	end		-- fail if isbn_str contains anything but digits, hyphens, or the uppercase X 	isbn_str = isbn_str:gsub( "-", "" ):gsub( " ", "" );	-- remove hyphens and spaces 	local len = isbn_str:len();   	if len ~= 10 and len ~= 13 then 		return false; 	end  	if len == 10 then 		if isbn_str:match( "^%d*X?$" ) == nil then 			return false 		end 		return is_valid_isbn(isbn_str, 10) 	else 		local temp = 0 		if isbn_str:match( "^97[89]%d*$" ) == nil 			then return false 		end	-- isbn13 begins with 978 or 979 		isbn_str = { isbn_str:byte(1, len) } 		for i, v in ipairs( isbn_str ) do 			temp = temp + (3 - 2*(i % 2)) * tonumber( string.char(v) ) 		end 		return temp % 10 == 0 	end end  function is_valid_isbn (isxn_str, len) 	local temp = 0 	isxn_str = { isxn_str:byte(1, len) }	-- make a table of bytes 	len = len + 1;							-- adjust to be a loop counter 	for i, v in ipairs( isxn_str ) do		-- loop through all of the bytes and calculate the checksum 		if v == string.byte( "X" ) then		-- if checkdigit is X 			temp = temp + 10*( len - i )	-- it represents 10 decimal 		else 			temp = temp + tonumber( string.char(v) ) * (len - i) 		end 	end 	return temp % 11 == 0					-- returns true if calculation result is zero end  function pmid(frame) 	return check(frame.args[1], 30000000) end  function rfc(frame) 	return check(frame.args[1], 10000) end  function check(id, test_limit) 	if id:match("[^%d]") then							-- if id has anything but digits 		return false 	else 		local id_num = tonumber(id)						-- convert id to a number for range testing 		if 1 > id_num or test_limit < id_num then		-- if id is outside test limit boundaries 			return false 		end 	end 	 	return true end  -- The code taken from Module:Citation/CS1 and changed  return {isbn = checkisbn, pmid = pmid, rfc = rfc}