Finally solved the mystery of why isNumeric doesn’t work how you might think

In Classic ASP there is the function isNumeric, which one uses to test if a value is a number. So for example:

Response.write IsNumeric("21")  // true
Response.write IsNumeric("David")  // false

Seems straightforward so far. Then you try this:

Response.write IsNumeric("15447D")  // true

Every so often this little quirk would popup. This morning it’s been driving me mad because I had a set of string values that were mixtures of alphanumeric, or numeric values. I wanted to discriminate between numbers and not-numbers and store the values in different columns in a database table. Using isNumeric though kept throwing errors – it tested 15447D as being a number, and of course once I tried to insert that into an integer column an error was thrown.

Finally figured how what’s happening after a little digging online. isNumeric also considers hex values to be valid numbers. 15447D is hex for 1393789. Now the lights go on – only wondered about this for 20 years.

Apparently this is also the case for octal values.

So a custom function along these lines is the answer:

Function myIsNumeric(ByVal Value)
	Dim regEx
	Set regEx = New RegExp
	regEx.pattern = "^(0|[1-9][0-9]*)$"
	myIsNumeric = Regex.Test(Value)
End Function

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