Reusing expression in asp.NET regex validation control

You can re-use your regex expression(extract it to the constant and set it in the Page_Load function for all required regex validators)

public const string WholeNumbersValidation = @"^\$?[0-9]+(\.[0]{2})?$";

protected void Page_Load(object sender, EventArgs e)
{
   AddHourlyRateRegexValidator.ValidationExpression = WholeNumbersValidation;
   AddHourlyRateRegexValidator.ErrorMessage= "Please enter a valid charge rate (whole number only)";
   validatorIdForNextField.ValidationExpression = WholeNumbersValidation;
   validatorIdForNextField.ErrorMessage= "Error message text 2";

}

, but you couldn't re-use one regex ASP.NET validator for a couple of fields, because do not accept more than one ID in the ControlToValidate property.

You could write your own client-side validation function that validates the textboxes all at once upon submit to resolve this issue and you should also have to write a validation function for the server side as well.

Have you considered adding the expression as a project-level setting, and then setting the "ValidationExpression" in code-behind?

You will have to use multiple validators, but you should be able to store the expression as a value in the web.config file, then access it via inline C#.

In web.config:

<appSettings>
  <add key="myRegExString" value="^\$?[0-9]+(\.[0]{2})?$" />
</appSettings>

In YourPage.aspx:

<asp:textbox ID="AddHourlyRate" runat="server" width="55px" />
<asp:RegularExpressionValidator  ID="AddHourlyRateRegexValidator"
    ErrorMessage="Please enter a valid charge rate (whole number only)" 
    ValidationExpression="<% = ConfigurationSettings.AppSettings["myRegExString"]%>" ControlToValidate="AddHourlyRate" 
    Display="Dynamic" runat="server" />

You can certainly do something like the following:

private RegularExpressionValidator GetValidatorControl(string controlToValidate, string errorMessage)
{

    RegularExpressionValidator validator = new RegularExpressionValidator();
    validator.ID = String.Format("{0}RegExValidator", controlToValidate);
    validator.ValidationExpression = @"^\$?[0-9]+(\.[0]{2})?$";
    validator.ControlToValidate = controlToValidate;
    validator.ErrorMessage = errorMessage;
    return validator;

}

And, in the Page_Load event or similar, you can call it this way:

Page.Controls.AddAt(Page.Controls.IndexOf(AddHourlyRate) + 1, GetValidatorControl(control.ID, "my validation message"));

This will add the validator control to your page and your expression will only be defined here (it can be placed into another class as a static method and shared throughout the project).

You could also use the method in this Stack Overflow answer to get all of the RegEx validators on your page, and then set the ValidationExpression on each one. For example, if you wrap all of your controls in a Panel (with an ID of RootPanel in my example) or similar container control then you could just:

IEnumerable<Control> validators = RootPanel.FlattenChildren();

IEnumerator<Control> enumerator = validators.GetEnumerator();

while (enumerator.MoveNext() == true)
{
    if (enumerator.Current is RegularExpressionValidator) //Only interested in RegularExpressionValidators. You could even filter this further by naming them consistently and checking for a fragment of the ID here.
    {
        ((RegularExpressionValidator)enumerator.Current).ValidationExpression = @"^\$?[0-9]+(\.[0]{2})?$"; //Load from config if necessary!
    }
}

To use the FlattenChildren method, place it into a public static class somewhere inside your project and reference it with the appropriate using directives. See this article for extension methods in .Net.

No matter what happens, you'll need one RegularExpressionValidator per control. Your other option, is, of course, to validate these manually yourself in javascript or on PostBack.