Parsing with Regular Expressions - IsDigit

by Les Smith and Brian Davis

How can I easily tell if the first character of a string is numeric?  There is no intrinsic function in .NET or VB6 for doing this.  Being an old Clipper programmer, when I moved to VB3, I wrote a VB function to replace the Clipper function IsDigit, which returns True if the first character of a string is numeric.  The code for that function is shown below.

Function IsDigit(ByVal cS As String) As Boolean       ' Returns True if first character of cString is digit,       ' otherwise False.       Dim cTemp As String       Dim vTemp As Integer       cTemp = cS.Substring(0, 1) ' get first character first       ' cant pass Asc() an empty string       If cTemp.Trim.Length = 0 Then          Return False       End If       vTemp = Asc(cTemp)       Return (vTemp > 47 AndAlso vTemp < 58)<BR>    End Function

In .NET, using a Regular Expression is much simpler, as shown by the code below.  Although you would have to run performance tests to determine if invoking the Regex Engine is slower or faster in this case, a knowledge of Regular Expressions will be a plus for any developer in the long run.

Public Function IsDigit(ByVal Source As String) As Boolean       ' Returns True if first character of Source is digit,       ' otherwise False.       ' the expression says to match the first character in the       ' string for numeric       Return Regex.IsMatch(Source, "^\d.*$", RegexOptions.Singleline)    End Function