Get the difference between 2 dates in RegEx (ASP Classic)

Hello All;

I need to get the different between 2 dates inside of a RegEx script.
The following script demonstrates one that works, and the other that does not work.

<%
Function Reggex(strString, strPattern, strReplace)

	Dim RE: Set RE = New RegExp

	With RE
		.Pattern = strPattern
		.Global = True
		.IgnoreCase = True
		Reggex = .Replace(strString, strReplace)
	End With
	
End Function

Function ReplaceChar(strString)
fromDate="1-Jan-2000"
toDate=date
strString = Reggex(strString, "\[BusDate\]", ""&DateDiff("yyyy",fromDate,toDate)&"") 

'This will give the output of 16 (being that it is 2016 now)
'However, what I need is to be able to type in the 2 dates.

strString = Reggex(strString, "\[theDate:([^\]]+)\|(.*?)\]", ""&DateDiff("yyyy","$1","$2")&"")

'However, the output in this example, will give me 1 (ONE)
ReplaceChar = strString
End Function
working = "[Busdate]"
testing = "[theDate:1950|2016]"
%>
Working: <%=ReplaceChar(working)%><br />
Non-working: <%=ReplaceChar(testing)%>

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I know there has to be a way to do this.
Any idea's?