Excel VBA: retain values of a column based on reported string

Sub Prefix_Retention()
    
    Dim strCol As String
    Dim lngLastRow As Long
    Dim lngRow As Long
    Dim intPos As Integer
    Dim strPrefix As String
    
    On Error GoTo Error_Routine
    
        strCol = InputBox("Please enter the column letter in which you want to remove the specific string", "Choose Column Letter")
        If strCol = vbNullString Then
            MsgBox "Unable to proceed, please properly report required information", vbCritical
            Exit Sub
        End If

        lngLastRow = Range(strCol & "1048576").End(xlUp).Row
        strPrefix = InputBox("Please enter the Prefix that you want to retain. It should be enter in capital letters", strPrefix)
        If strPrefix = vbNullString Then
            MsgBox "Unable to proceed, please properly report required information", vbCritical
            Exit Sub
        End If

    For lngRow = 1 To lngLastRow
        intPos = InStr(1, UCase(Cells(lngRow, strCol).Value), strPrefix)
        If intPos > 0 Then
            Cells(lngRow, strCol).Value = Trim(Mid(Cells(lngRow, strCol).Value, intPos))
        End If
    Next
    
    Exit Sub
Error_Routine:
    MsgBox Err.Description, vbExclamation, "Something went wrong!"
    
End Sub

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 strStartChar = Split(strInput, ";")(0)
    strEndChar = Split(strInput, ";")(1)
    
    strIO = InputBox("Please enter the column letters separated by a colon for the data in the form 'InputColum:OutputColumn'", "Choose Column Letters", "A:B")
        
    strColInput = Split(strIO, ":")(0)
    strColOutput = Split(strIO, ":")(1)
    
    If (Not IsValidColumnLetter(strColInput)) Or (Not IsValidColumnLetter(strColOutput)) Then
        MsgBox "You entered Invalid Column Letters.", vbExclamation
        Exit Sub
    End If
    
    lngLastRow = Range(strColInput & "1048576").End(xlUp).Row
    
    For lngRow = 1 To lngLastRow
        Cells(lngRow, strColOutput).Value = getString(Cells(lngRow, strColInput).Value, strStartChar, strEndChar)
    Next
    

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