Really, just use Decimal.TryParse
.
If the user uses a different culture where .
is not the decimal seperator, your code breaks anyway.
Example:
Public Shared Function CheckDecimalValue(txtbox As TextBox) As Boolean
Dim tmp As Decimal
Return Decimal.TryParse(txtbox.Text, tmp) AndAlso tmp >= 5 AndAlso tmp <= 14
End Function