strip characters

A file is open as Random.  I need to look at a field and strip the alphas out so all I have left are numerics.  I have created a user defined type, and can get to the field this way.  I am at a loss as to how to get the program to read one character at a time and get rid of the letters.  
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Dear LJ
          This best  method would be to read the entire file into a string .

In the first parse of string if ascii value of the character is less than 48 or its freater than 57 then at that substitute the character by space.

In the second parse read the string and if character is not space then write it into the same file and save it. That should give you the result

If you have any doubts email to me at
see the following
A subroutine that will translate out any character and replace it with any other character string:

Public Function SubTran$(Inst$, lookfor$, ByVal Change2$)
 Dim i, j As Integer
 Dim InString$
 InString$ = Inst$
 j = 1
 i = InStr(j, InString$, lookfor$)
 While i > 0
   InString$ = Left$(InString$, i - 1) + Change2$ + Mid$(InString$, i + Len(lookfor$))
   j = i + Len(Change2$)
   i = InStr(j, InString$, lookfor$)
 SubTran$ = InString$
End Function
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for i=1 to len(fld)
  if ch >= 48 and ch <= 57 then result=result & chr$(ch)
next i

Result should ony contain numerics after this routine.
A vb version of the translate instruction:

Public Trn$
'somewhere do once up front (form load? read from file?)
'set up translate table
dim i as integer
trn$=space$(256)  'default to spaces when not a number
for i=48 to 57  'Ascii 0 to Ascii 9
next i

'then to translate

'then a subroutine
public function xlt$(instr$)
dim x$
dim i as integer
for i =1 to len(x$)
next i
end function
or, what sbmc sez, cause he typed it in before I got around to it, but that was next, but only if you want to just concatenate all numerics together.  The key is knowing where in the Ascii range of 0 to 255 the number characters fall.  

On a vb functional basis, there is always the IsNumeric function, but that carries a load of code around to check for numeric in an entire string.
LJ083198Author Commented:
It is not that this answer is unsatisfactory, I just used the solution provided by sbmc.  It seemed much more straightforward, and it worked. As far as sbmc answer goes, I thought it was an A.
There's also some function API that you can use wich are isalpha()  and isnumeric()  if you need more info let me know about it.
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