https://learn.microsoft.com/en-us/dotnet/api/system.web.ui.webcontrols.regularexpressionvalidator.validationexpression?view=netframework-4.8
RegularExpressionValidator.ValidationExpression Property (System.Web.UI.WebControls) | Microsoft...
Gets or sets the regular expression that determines the pattern used to validate a field.
property systemweb uimicrosoft