Как сделать evaluate формулы, которая введена в TextBox?

Private Sub Command1_Click ()
    Dim n As Double
    If e_eval(Text1.Text, n) Then
      MsgBox n
    End If
End Sub
Add the following code in the (General)(Declaration) section of Form1:
 
Dim e_input As String     ' Expression input string.
Dim e_tok As String       ' Current token kind.
Dim e_spelling As String  ' Current token spelling.
Dim e_error As Integer    ' Tells if syntax error occurred.
 
' e_eval
'   Evaluate a string containing an infix numeric expression.
'   If successful, return true and place result in .
'   This is the top-level function in the expression evaluator.
Function e_eval (ByVal s As String, value As Double) As Integer
   ' Initialize.
   e_error = 0
   e_input = s
   Call e_nxt
 
   ' Evaluate.
   value = e_prs(1)
 
   ' Check for unrecognized input.
   If e_tok <> '' And Not e_error Then
      MsgBox 'syntax error, token = '' + e_spelling + '''
      e_error = -1
   End If
 
   e_eval = Not e_error
End Function
 
' e_prs
'   Parse an expression, allowing operators of a specified
'   precedence or higher. The lowest precedence is 1.
'   This function gets tokens with e_nxt and recursively
'   applies operator precedence rules.
Function e_prs (p As Integer) As Double
   Dim n As Double    ' Return value.
   Dim fun As String  ' Function name.
 
   ' Parse expression that begins with a token (precedence 12).
   If e_tok = 'num' Then
      ' number.
      n = Val(e_spelling)
      Call e_nxt
   ElseIf e_tok = '-' Then
      ' unary minus.
      Call e_nxt
      n = -e_prs(11)    ' Operand precedence 11.
   ElseIf e_tok = 'not' Then
      ' logical NOT.
      Call e_nxt
      n = Not e_prs(6)  ' Operand precedence 6.
   ElseIf e_tok = '(' Then
      ' parentheses.
      Call e_nxt
      n = e_prs(1)
      Call e_match(')')
   ElseIf e_tok = 'id' Then
      ' Function call.
      fun = e_spelling
      Call e_nxt
      Call e_match('(')
      n = e_prs(1)
      Call e_match(')')
      n = e_function(fun, n)
   Else
      If Not e_error Then
         MsgBox 'syntax error, token = '' + e_spelling + '''
         e_error = -1
      End If
              Loop While is_id%
          e_input = c + e_input
    
          ' Check for keyword.
          is_keyword = -1
          Select Case LCase$(e_spelling)
             Case 'and'
             Case 'eqv'
             Case 'imp'
             Case 'mod'
             Case 'not'
             Case 'or'
             Case 'xor'
             Case Else: is_keyword = 0
          End Select
          If is_keyword Then
             e_tok = LCase$(e_spelling)
          End If
    
       ' Check for <=, >=, <>.
       Case '<', '>'
          e_tok = c
          c = Left$(e_input, 1)
          If c = '=' Or c = '>' Then
             e_tok = e_tok + c
             e_input = Mid$(e_input, 2)
          End If
    
       ' Single character token.
       Case Else
          e_tok = c
   End Select
 
   If e_spelling = '' Then
       e_spelling = e_tok
   End If
End Sub
 
' e_match
'   Check the current token and skip past it.
'   This function helps with syntax checking.
Sub e_match (token As String)
   If Not e_error And e_tok <> token Then
      MsgBox 'expected ' + token + ', got '' + e_spelling + '''
      e_error = -1
   End If
   Call e_nxt
End Sub