Message
 This is my final, cleaned up version. It looks a bit neater in places, and handles numbers up to 10^66.
 Convert a number to words
 Author: Nick Gammon
 Date: 18th March 2010
 Examples of use:
 words = convert_numbers_to_words ("94921277802687490518")
 number = convert_words_to_numbers ("one hundred eight thousand three hundred nine")
 Both functions return nil and an error message so you can check for failure,
 or assert, eg. words = assert (convert_numbers_to_words ("2687490518"))
 Units, must be in inverse order!
 The trailing space is required as the space between words
local inverse_units = {
"vigintillion ",  10^63
"novemdecillion ",  10^60
"octodecillion ",  10^57
"septendecillion ",  10^54
"sexdecillion ",  10^51
"quindecillion ",  10^48
"quattuordecillion ", 10^45
"tredecillion ",  10^42
"duodecillion ",  10^39
"undecillion ",  10^36
"decillion ",  10^33
"nonillion ",  10^30
"octillion ",  10^27
"septillion ",  10^24
"sextillion ",  10^21
"quintillion ",  10^18
"quadrillion ",  10^15
"trillion ",  10^12
"billion ",  10^9
"million ",  10^6
"thousand ",  10^3
}  inverse_units
local inverse_numbers = {
"one ",
"two ",
"three ",
"four ",
"five ",
"six ",
"seven ",
"eight ",
"nine ",
"ten ",
"eleven ",
"twelve ",
"thirteen ",
"fourteen ",
"fifteen ",
"sixteen ",
"seventeen ",
"eighteen ",
"nineteen ",
"twenty ",
[30] = "thirty ",
[40] = "forty ",
[50] = "fifty ",
[60] = "sixty ",
[70] = "seventy ",
[80] = "eighty ",
[90] = "ninety ",
}  inverse_numbers
local function convert_up_to_999 (n)
if n <= 0 then
return ""
end  if zero
local hundreds = math.floor (n / 100)
local tens = math.floor (n % 100)
local result = ""
 if over 99 we need to say x hundred
if hundreds > 0 then
result = inverse_numbers [hundreds] .. "hundred "
if tens == 0 then
return result
end  if only a digit in the hundreds column
 to have "and" between things like "hundred and ten"
 uncomment the next line
 result = result .. "and "
end  if
 up to twenty it is then just five hundred (and) fifteen
if tens <= 20 then
return result .. inverse_numbers [tens]
end  if
 otherwise we need: thirty (something)
result = result .. inverse_numbers [math.floor (tens / 10) * 10]
 get final digit (eg. thirty four)
local digits = math.floor (n % 10)
 to put a hyphen between things like "fortytwo"
 uncomment the WITH HYPHEN line and
 comment out the NO HYPHEN line
if digits > 0 then
result = result .. inverse_numbers [digits]  NO HYPHEN
 result = string.sub (result, 1, 2) .. "" .. inverse_numbers [digits]  WITH HYPHEN
end  if
return result
end  convert_up_to_999
 convert a number to words
 See: http://www.gammon.com.au/forum/?id=10155
function convert_numbers_to_words (n)
local s = tostring (n)
 preliminary sanity checks
local c = string.match (s, "%D")
if c then
return nil, "Nonnumeric digit '" .. c .. "' in number"
end  if
if #s == 0 then
return nil, "No number supplied"
elseif #s > 66 then
return nil, "Number too big to convert to words"
end  if
 make multiple of 3
while #s % 3 > 0 do
s = "0" .. s
end  while
local result = ""
local start = #inverse_units  (#s / 3) + 2
for i = start, #inverse_units do
local group = tonumber (string.sub (s, 1, 3))
if group > 0 then
result = result .. convert_up_to_999 (group) .. inverse_units [i]
end  if not zero
s = string.sub (s, 4)
end  for
result = result .. convert_up_to_999 (tonumber (s))
if result == "" then
result = "zero"
end  if
return (string.gsub (result, " +$", ""))  trim trailing spaces
end  convert_numbers_to_words
 Convert words to a number
 Author: Nick Gammon
 Date: 18th March 2010
 Does NOT handle decimal places (eg. four point six)
local numbers = {
zero = bc.number (0),
one = bc.number (1),
two = bc.number (2),
three = bc.number (3),
four = bc.number (4),
five = bc.number (5),
six = bc.number (6),
seven = bc.number (7),
eight = bc.number (8),
nine = bc.number (9),
ten = bc.number (10),
eleven = bc.number (11),
twelve = bc.number (12),
thirteen = bc.number (13),
fourteen = bc.number (14),
fifteen = bc.number (15),
sixteen = bc.number (16),
seventeen = bc.number (17),
eighteen = bc.number (18),
nineteen = bc.number (19),
twenty = bc.number (20),
thirty = bc.number (30),
forty = bc.number (40),
fifty = bc.number (50),
sixty = bc.number (60),
seventy = bc.number (70),
eighty = bc.number (80),
ninety = bc.number (90),
}  numbers
local units = {
hundred = bc.number ("100"),
thousand = bc.number ("1" .. string.rep ("0", 3)),
million = bc.number ("1" .. string.rep ("0", 6)),
billion = bc.number ("1" .. string.rep ("0", 9)),
trillion = bc.number ("1" .. string.rep ("0", 12)),
quadrillion = bc.number ("1" .. string.rep ("0", 15)),
quintillion = bc.number ("1" .. string.rep ("0", 18)),
sextillion = bc.number ("1" .. string.rep ("0", 21)),
septillion = bc.number ("1" .. string.rep ("0", 24)),
octillion = bc.number ("1" .. string.rep ("0", 27)),
nonillion = bc.number ("1" .. string.rep ("0", 30)),
decillion = bc.number ("1" .. string.rep ("0", 33)),
undecillion = bc.number ("1" .. string.rep ("0", 36)),
duodecillion = bc.number ("1" .. string.rep ("0", 39)),
tredecillion = bc.number ("1" .. string.rep ("0", 42)),
quattuordecillion = bc.number ("1" .. string.rep ("0", 45)),
quindecillion = bc.number ("1" .. string.rep ("0", 48)),
sexdecillion = bc.number ("1" .. string.rep ("0", 51)),
septendecillion = bc.number ("1" .. string.rep ("0", 54)),
octodecillion = bc.number ("1" .. string.rep ("0", 57)),
novemdecillion = bc.number ("1" .. string.rep ("0", 60)),
vigintillion = bc.number ("1" .. string.rep ("0", 63)),
}  units
 convert a number in words to a numeric form
 See: http://www.gammon.com.au/forum/?id=10155
 Thanks to David Haley
function convert_words_to_numbers (s)
local stack = {}
local previous_type
for word in string.gmatch (s:lower (), "[%a%d]+") do
if word ~= "and" then  skip "and" (like "hundred and fifty two")
local top = #stack
 If the current word is a number (English or numeric),
 and the previous word was also a number, pop the previous number
 from the stack and push the addition of the two numbers.
 Otherwise, push the new number.
local number = tonumber (word)  try for numeric (eg. 22 thousand)
if number then
number = bc.number (number)  turn into "big number"
else
number = numbers [word]
end  if a numberword "like: twenty"
if number then
if previous_type == "number" then  eg. forty three
local previous_number = table.remove (stack, top)  get the forty
number = number + previous_number  add three
end  if
table.insert (stack, number)
previous_type = "number"
else
 If the current word is a unit, multiply the number on the top of the stack by the unit's magnitude.
local unit = units [word]
if not unit then
return nil, "Unexpected word: " .. word
end  not unit
previous_type = "unit"
 It is an error to get a unit before a number.
if top == 0 then
return nil, "Cannot have unit before a number: " .. word
end  starts of with something like "thousand"
 pop until we get something larger on the stack
local interim_result = bc.number (0)
while top > 0 and stack [top] < unit do
interim_result = interim_result + table.remove (stack, top)
top = #stack
end  while
table.insert (stack, interim_result * unit)
end  if number or not
end  if 'and'
end  for each word
if #stack == 0 then
return nil, "No number found"
end  nothing
 When the input has been parsed, sum all numbers on the stack.
local result = bc.number (0)
for _, item in ipairs (stack) do
result = result + item
end  for
return result
end  function convert_words_to_numbers
The test bed you can use to confirm it works is this (just put underneath and run it):
print (string.rep ("", 75))
local start_time = GetInfo (232)
for i = 1, 5000 do
local s = ""
for i = 1, math.floor (MtRand () * 66 + 1) do
s = s .. math.floor (MtRand () * 10)
end  for
print ("Converting: '" .. s .. "'")
local words = assert (convert_numbers_to_words (s))
print ("Result =", words)
n = assert (convert_words_to_numbers (words))
print ("Result : '" .. n:tostring () .. "'", "length =", #bc.tostring (n))
if not bc.iszero (n) then
assert (string.gsub (s, "^0+", "") == n:tostring (), "Conversion failed!")
end  if
end  for
local end_time = GetInfo (232)
print (string.format ("Time taken = %0.3f seconds", end_time  start_time))
The code handles up to 999 vigintillion, and has a few sanity checks on the supplied number when doing convert_numbers_to_words (eg. is it a number, is it too long).
Example output:
Converting: '221854371162103637052189353391241767991981161850420402320763181735'
Result = two hundred twenty one vigintillion eight hundred fifty four novemdecillion three hundred seventy one octodecillion one hundred sixty two septendecillion one hundred three sexdecillion six hundred thirty seven quindecillion fifty two quattuordecillion one hundred eighty nine tredecillion three hundred fifty three duodecillion three hundred ninety one undecillion two hundred forty one decillion seven hundred sixty seven nonillion nine hundred ninety one octillion nine hundred eighty one septillion one hundred sixty one sextillion eight hundred fifty quintillion four hundred twenty quadrillion four hundred two trillion three hundred twenty billion seven hundred sixty three million one hundred eighty one thousand seven hundred thirty five
Result : '221854371162103637052189353391241767991981161850420402320763181735' length = 66
The comments in the code indicate the two changes needed if you want hyphens in generated words (eg. twentytwo) or the word "and" (eg. hundred and five).
The code does not handle negative numbers or decimals. You could easily add a wrapper to do that.
The code to convert words to numbers just looks for words and digits (thus skipping hyphens). It would also skip other nonletter/nondigit things (like commas) so if you are processing untrusted input you may want to check for those yourself.
Examples of use:
print (convert_numbers_to_words ("94921277802687490518"))
(Large numbers need to be quoted, otherwise Lua turns them into scientific notation, like "9.4921277802687e+019", which it won't handle).
Result:
ninety four quintillion nine hundred twenty one quadrillion two hundred seventy seven trillion eight hundred two billion six hundred eighty seven million four hundred ninety thousand five hundred eighteen
And converting back:
print (convert_words_to_numbers ("sixty five quintillion five hundred fifty nine quadrillion eight hundred eighty seven trillion seven hundred seventy one billion one hundred sixty six million one hundred eight thousand three hundred nine"))
Result:
65559887771166108309

 Nick Gammon
www.gammon.com.au, www.mushclient.com  top 
