Next if the number is greater than 99, the function gets the hundreds digit and uses the one_to_nineteen array to look up its string representation. It adds that representation plus the word "hundred" to the result. If the number has no other digits, the function returns what it has so far.

Next if the remaining number is less than 20, the function uses the one_to_nineteen array to look up the number's representation.

If the number is greater than 19, the function gets the tens digit and uses the multiples_of_ten array to get its representation. It then uses the one_to_nineteen array to add on the representation of the ones digit.

' Convert a number between 0 and 999 into words.
Private Function GroupToWords(ByVal num As Integer) As _
    String
Static done_before As Boolean
Static one_to_nineteen() As String
Static multiples_of_ten() As String

Dim digit As Integer
Dim result As String

    If Not done_before Then
        done_before = True
        one_to_nineteen = Split("zero,one,two,three," & _
            "four,five,six,seven,eight,nine,ten," & _
            "eleven,twelve,thirteen,fourteen,fifteen," & _
            "sixteen,seventeen,eightteen,nineteen", ",")
        multiples_of_ten = Split("twenty,thirty," & _
            "forty,fifty,sixty,seventy,eighty,ninety", ",")
    End If

    ' If the number is 0, return an empty string.
    GroupToWords = 0
    If num = 0 Then Exit Function

    ' Handle the hundreds digit.
    result = ""
    If num > 99 Then
        digit = num \ 100
        num = num Mod 100
        result = one_to_nineteen(digit) & " hundred"
    End If

    ' If num = 0, we have hundreds only.
    If num = 0 Then
        GroupToWords = Trim$(result)
        Exit Function
    End If

    ' See if the rest is less than 20.
    If num < 20 Then
        ' Look up the correct name.
        result = result & " " & one_to_nineteen(num)
    Else
        ' Handle the tens digit.
        digit = num \ 10
        num = num Mod 10
        result = result & " " & multiples_of_ten(digit - 2)

        ' Handle the final digit.
        If num > 0 Then
            result = result & " " & one_to_nineteen(num)
        End If
    End If

    GroupToWords = Trim$(result)
End Function

First the function checks its use_us_group_names parameter and initializes its groups array to a list of group names that is either appropriate for US/scientific use or for non-US use.

Next the function cleans up its input string a bit. It removes "$", "," and leading zeros. If the string contains a decimal point, it removes anything after it.

The code then determines how many groups of three digits the number needs. It pads the string on the left so its length is a multiple of 3.

Now the code steps through the number's three-digit groups. For each group, it extracts the digits, converts them into a number, uses function GroupToWords to convert the group into a string, and adds the group's name to the result.

Finally the function removes the trailing ", " if present and returns the result.

' Return a word representation of the whole number value.
Private Function NumberToString(ByVal num_str As String, _
    Optional ByVal use_us_group_names As Boolean = True) As _
    String
Const CURRENCY_CHAR As String = "$"
Const SEPARATOR As String = ","
Const DECIMAL_POINT As String = "."

Dim groups() As String
Dim pos As Integer
Dim num_groups As Integer
Dim result As String
Dim group_num As Integer
Dim group_str As String
Dim group_value As Integer

    ' Get the appropiate group names.
    If use_us_group_names Then
        groups = Split(",thousand,million,billion," & _
            "trillion,quadrillion,quintillion," & _
            "sextillion,septillion,octillion," & _
            "nonillion,decillion,undecillion," & _
            "duodecillion,tredecillion," & _
            "quattuordecillion,quindecillion," & _
            "sexdecillion,septendecillion," & _
            "octodecillion,novemdecillion," & _
            "vigintillion", ",")
    Else
        groups = Split(",thousand,million," & _
            "milliard,billion,1000 billion," & _
            "trillion,1000 trillion,quadrillion," & _
            "1000 quadrillion,quintillion," & _
            "1000 quintillion,sextillion," & _
            "1000 sextillion,septillion," & _
            "1000 septillion,octillion," & _
            "1000 octillion,nonillion," & _
            "1000 nonillion,decillion," & _
            "1000 decillion", ",")
    End If

    ' Clean the string a bit.
    ' Remove "$", ",", leading zeros, and
    ' anything after a decimal point.
    num_str = Replace$(num_str, CURRENCY_CHAR, "")
    num_str = Replace$(num_str, SEPARATOR, "")
    Do While Left$(num_str, 1) = "0"
        num_str = Mid$(num_str, 2)
    Loop
    pos = InStr(num_str, DECIMAL_POINT)
    If pos = 1 Then
        NumberToString = "zero"
        Exit Function
    ElseIf pos > 1 Then
        num_str = Left$(num_str, pos - 1)
    End If

    ' See how many groups there will be.
    num_groups = (Len(num_str) + 2) \ 3

    ' Pad so length is a multiple of 3.
    num_str = Space$(num_groups * 3 - Len(num_str)) & _
        num_str

    ' Process the groups, largest first.
    result = ""
    For group_num = num_groups - 1 To 0 Step -1
        ' Get the next three digits.
        group_str = Mid$(num_str, 1, 3)
        num_str = Mid$(num_str, 4)
        group_value = CInt(group_str)

        ' Convert the group into words.
        If group_value > 0 Then
            If group_num >= UBound(groups) Then
                result = result & GroupToWords(group_value) _
                    & _
                    " ?, "
            Else
                result = result & GroupToWords(group_value) _
                    & _
                    " " & groups(group_num) & ", "
            End If
        End If
    Next group_num

    ' Remove the trailing ", ".
    If Right$(result, 2) = ", " Then
        result = Left$(result, Len(result) - 2)
    ElseIf Len(result) = 0 Then
        result = "zero"
    End If

    NumberToString = Trim$(result)
End Function

txtResult.Text = _
    NumberToString( _
        txtNumber.Text, _
        chkUseUsGroupNames.Value = vbChecked)