Regular Expression to allow Enter Key in MultiLine TextBox in ASP.Net

HiSumeet,

You should add special character for new line into regex pattern.

Check this example. Now please take its reference and correct your code.

HTML

<asp:TextBox runat="server" ID="txtaddress" TextMode="MultiLine" />
<asp:RegularExpressionValidator Display="Dynamic" ID="RegularExpressionValidator1txtaddress"
    ControlToValidate="txtaddress" runat="server" ErrorMessage="Only alphabet & space allowed. Max. 300 chars"
    ValidationGroup="reg" CssClass="valmsg" ValidationExpression="[a-zA-Z \r\n]{1,300}$"></asp:RegularExpressionValidator>
<asp:Button Text="Save" runat="server" ValidationGroup="reg" />

Screenshot