<asp:regularexpressionvalidator id="regExpDate" 
runat="server" 

ValidationExpression="^\d{2}[\/-]\d{2}[\/-]\d{2,4}$"

ErrorMessage="It is not a valid date" 
ControlToValidate="txtpatientvisitdate" 
EnableClientScript="False" 
Display="Dynamic"></asp:regularexpressionvalidator>

Kamal..

<asp:RegularExpressionValidator ID="revEndDate" 
ControlToValidate="txtEndDate" ValidationExpression="((^
(10|12|0?[13578])([/])(3[01]|[12][0-9]|0?[1-9])([/])((1[8-9]
\d{2})|([2-9]\d{3}))$)|(^(11|0?[469])([/])(30|[12][0-9]|0?
[1-9])([/])((1[8-9]\d{2})|([2-9]\d{3}))$)|(^(0?2)([/])(2[0-
8]|1[0-9]|0?[1-9])([/])((1[8-9]\d{2})|([2-9]\d{3}))$)|(^(0?
2)([/])(29)([/])([2468][048]00)$)|(^(0?2)([/])(29)([/])
([3579][26]00)$)|(^(0?2)([/])(29)([/])([1][89][0][48])$)|(^
(0?2)([/])(29)([/])([2-9][0-9][0][48])$)|(^(0?2)([/])(29)
([/])([1][89][2468][048])$)|(^(0?2)([/])(29)([/])([2-9][0-9]
[2468][048])$)|(^(0?2)([/])(29)([/])([1][89][13579][26])$)|
(^(0?2)([/])(29)([/])([2-9][0-9][13579][26])$))" 
SetFocusOnError="True" Display="Dynamic" 
runat="server">*</asp:RegularExpressionValidator>

Whereas in ASP.Net we can use RegularExpression Validator to
validate the date value.  But in VB.NET we have to use Class
RegEx.  

If Regex.IsMatch(txtDate,
"(0[1-9]|[12][0-9]|3[01])-(0[1-9]|1[012])-((19|20)\d\d)") Then