If you are using VB .Net then take a look at this code.
Dim sampleUserInput As String = "8, 5, 12,12, 15, a,99,52" 'with errors
Dim theLetters As String = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
'
'step 1 separate the user input
Dim seps() As Char = New Char() {","c, " "c}
Dim nums() As String = sampleUserInput.Split(seps, StringSplitOptions.RemoveEmptyEntries)
'step 2 look at each number and use it
'to get the letter
Dim cnvrtd As New System.Text.StringBuilder
Dim idx As Integer
For Each s As String In nums 'look at each number
'convert string to number and check that it is in range
If Integer.TryParse(s, idx) AndAlso idx <= theLetters.Length Then
cnvrtd.Append(theLetters(idx - 1)) 'use the number to get a character
End If
Next
Debug.WriteLine(cnvrtd.ToString)