ValidationSummary collect all the error in your input and display.
So indirectly you have answerd your question by yourself in your question
You just don't know the syntax i think.Here it is:
If you have some collection of input in aspx,you have defined also the regular expression for specific input.For example:
<div>
<asp:TextBox ID="txt" runat="server" MaxLength="100"></asp:TextBox>
<asp:RegularExpressionValidator ID="revtxt" runat="server"SetFocusOnError="true"ErrorMessage="Please enter correct txt" ControlToValidate="txt" ValidationGroup="Submit"
ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*">*</asp:RegularExpressionValidator>
<div>
<div>
<asp:TextBox ID="txt1" runat="server" MaxLength="100"></asp:TextBox>
<asp:RegularExpressionValidator ID="revtxt1" runat="server"SetFocusOnError="true"ErrorMessage="Please enter correct txt1" ControlToValidate="txtEmail" ValidationGroup="Submit"
ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*">*</asp:RegularExpressionValidator>
<div>
<div>
<asp:Button ID="btnSubmit" runat="server" Text="Save" ValidationGroup="Submit" OnClick="btnSubmit_Click" />
</div>
<div>
<asp:ValidationSummary ID="ValidationSummary"runat="server"ValidationGroup="Submit" />
</div>
So in every input or button you have to define ValidationGroup attribute
Else if you want to check in codebehind if all of this input are validated you have to do this:
if(Page.IsValid)
{
//your code here
}