Validating Expressions: The RegularExpressionValidator Control

Validating Expressions: The `RegularExpressionValidator` Control

You can use RegularExpressionValidator to match the value entered into a form field to a regular expression. You can use this control to check whether a user has entered, for example, a valid e-mail address, telephone number, or username or password. Samples of how to use a regular expression to perform all these validation tasks are provided in the following sections. The properties and methods of this control are listed in Table 3.2.

Table 3.2. `RegularExpressionValidator` Properties, Methods, and Events

Properties	Description
`ControlToValidate`	Specifies the ID of the control that you want to validate.
`Display`	Sets how the error message contained in the `Text` property is displayed. Possible values are `Static` , `Dynamic` , and `None` ; the default value is `Static` .
`EnableClientScript`	Enables or disables client-side form validation. This property has the value `True` by default.
`Enabled`	Enables or disables both server and client-side validation. This property has the value `True` by default.
`ErrorMessage`	Specifies the error message that is displayed in the `ValidationSummary` control. This error message is displayed by the control when the `Text` property is not set.
`IsValid`	Has the value `True` when the validation check succeeds and `False` otherwise .
`Text`	Sets the error message displayed by the control.
`ValidationExpression`	Specifies the regular expression to use when performing validation.
Methods	Description
`Validate`	Performs validation and updates the `IsValid` property.
Events	Description
None

NOTE

Regular expressions are discussed in detail in Chapter 24, "Working with Collections and Strings."

You assign the regular expression that you want to use when performing validation to the ValidationExpression property. You can perform complex types of validation by using the correct regular expression.

CAUTION

Regular expressions work somewhat differently in JavaScript than they do in the .NET framework. So, in certain circumstances, the client-side validation code used for matching regular expressions might return different results than the server-side validation code. Even worse , a valid .NET regular expression might generate a JavaScript error. For example, the syntax for using regular expression options inline, such as the i option, differs between JavaScript and the .NET classes.

You can submit the form in Listing 3.4, for example, only if you enter a product code that starts with the uppercase letter P and contains no more, or no fewer, than four numerals in a row. The RegularExpressionValidator control uses the regular expression P[0-9]{4} .

Listing 3.4 `RegularExpressionValidator.aspx`

 <Script Runat="Server"> Sub Button_Click( s As Object, e As EventArgs )   If IsValid Then     Response.Redirect( "ThankYou.aspx" )   End If End Sub </Script> <html> <head><title>RegularExpressionValidator.aspx</title></head> <body> <form Runat="Server"> Product Code: <br> <asp:TextBox   id="txtProductCode"   Runat="Server"/> <asp:RegularExpressionValidator   ControlToValidate="txtProductCode"   Text="Invalid Product Code!"   ValidationExpression="P[0-9]{4}"   Runat="Server" /> <p> <asp:Button   Text="Submit"   OnClick="Button_Click"   Runat="Server"/> </form> </body> </html>

The C# version of this code can be found on the CD-ROM.

If you experiment with the page contained in Listing 3.4, you'll quickly discover that you can submit the form without entering any text into the text box. The RegularExpressionValidator control does not require a value.

The only way to require a value is to combine RegularExpressionValidator with RequiredFieldValidator . Nothing prevents you from associating multiple Validator controls with the same control.

Validating E-Mail Addresses

One of the most common and difficult validation tasks that arise when performing form validation is validating an e-mail address. This task is actually much more difficult than you might assume because the e-mail standard is so complicated.

NOTE

You can find the specification for e-mail addresses in RFC 822, Standard for ARPA Internet Text Messages, at the following location:

 ftp://ftp.rfc-editor.org/in-notes/rfc822.txt

Even if a perfect e-mail validation regular expression is beyond your grasp, however, you can still hope to validate simple e-mail addresses. For example, you can use the following regular expression to check that an e-mail address starts with one or more nonwhitespace characters , followed by an @ sign, followed by one or more nonwhitespace characters, followed by a period, followed by one or more nonwhitespace characters:

 \S+@\S+\.\S+

So, this regular expression would match the following e-mail addresses:

steve@somewhere.com

steve@host.somewhere.com

steve_smith@somewhere.com

However, it would fail to match e-mail addresses like

 steve@aol steve smith@aol

which is what you want.

If you want to exclude all e-mail addresses that do not end with a top-level domain name between two and three characters, such as .com , .net , and .ws , you would use this expression:

 \S+@\S+\.\S{2,3}

The page in Listing 3.5 demonstrates how you would use this regular expression with the RegularExpressionValidator control.

Listing 3.5 `RegularExpressionValidatorEmail.aspx`

 <Script Runat="Server"> Sub Button_Click( s As Object, e As EventArgs )   If IsValid Then     Response.Redirect( "ThankYou.aspx" )   End If End Sub </Script> <html> <head><title>RegularExpressionvalidatorEmail.aspx</title></head> <body> <form Runat="Server"> Email Address: <br> <asp:TextBox   id="txtEmail"   Columns="50"   Runat="Server"/> <asp:RegularExpressionValidator   ControlToValidate="txtEmail"   Text="Invalid Email Address!"   ValidationExpression="\S+@\S+\.\S{2,3}"   Runat="Server" /> <p> <asp:Button   Text="Submit"   OnClick="Button_Click"   Runat="Server"/> </form> </body> </html>

The C# version of this code can be found on the CD-ROM.

Validating Usernames and Passwords

Typically, Web sites require you to enter a username and password containing only alphanumeric characters or the underscore character. You can perform this type of validation using a regular expression that looks like this:

\w+

This regular expression matches any expression that contains one or more word characters (a word character can be a letter, number, or the underscore character).

You also can specify a minimum and maximum length for a password by using a regular expression that looks like this:

 \w{8,20}

This regular expression matches only expressions that are between 8 and 20 characters long.

Finally, some Web sites require you to use at least one number and one letter in your password. You can use the following regular expression to satisfy this requirement:

 [a-zA-Z]+\w*\d+\w*

This regular expression requires you to enter at least one letter, followed by any number of word characters, followed by at least one number, followed by any number of word characters.

Listing 3.6 demonstrates how you can combine two RegularExpressionValidator controls and a RequiredFieldValidator control to validate a password. The Validation controls require you to enter a password that starts with at least one letter and contains one number and between 3 and 20 characters (special characters, such as # and ? , are not allowed).

Listing 3.6 `RegularExpressionValidatorPassword.aspx`

 <Script Runat="Server"> Sub Button_Click( s As Object, e As EventArgs )   If IsValid Then     Response.Redirect( "thankyou.aspx" )   End If End Sub </Script> <html> <head><title>RegularExpressionValidatorPassowrd.aspx</title></head> <body> <form Runat="Server"> Password: <br> <asp:TextBox   id="txtPassword"   Columns="30"   Runat="Server"/> <asp:RequiredFieldValidator   ControlToValidate="txtPassword"   Display="Dynamic"   Text="You must enter a password!"   Runat="Server" /> <asp:RegularExpressionValidator   ControlToValidate="txtPassword"   Display="Dynamic"   Text="Your password must contain between 3 and 20 characters!"   ValidationExpression="\w{3,20}"   Runat="Server" /> <asp:RegularExpressionValidator   ControlToValidate="txtPassword"   Display="Dynamic"   Text="Your password must contain at least one number and letter!"   ValidationExpression="[a-zA-Z]+\w*\d+\w*"   Runat="Server" /> <p> <asp:Button   Text="Submit"   OnClick="Button_Click"   Runat="Server"/> </form> </body> </html>

The C# version of this code can be found on the CD-ROM.

Validating Phone Numbers

Phone numbers are difficult to validate ” especially when you take into consideration foreign area codes and phone number extensions. Even if you ignore these problems and concentrate on U.S. phone numbers without extensions, many different formats still are used when entering a phone number. For example, the following formats are all commonly used:

 (555) 555-5555 555.555.5555 555 555-555

You can create a regular expression that matches all three of the preceding expressions, as follows :

 \(?\s*\d{3}\s*[\)\.\-]?\s*\d{3}\s*[\-\.]?\s*\d{4}

The page in Listing 3.7 illustrates how you can use this regular expression with RegularExpressionValidator .

Listing 3.7 `RegularExpressionValidatorPhone.aspx`

 <Script Runat="Server"> Sub Button_Click( s As Object, e As EventArgs )   If IsValid Then     Response.Redirect( "ThankYou.aspx" )   End If End Sub </Script> <html> <head><title>RegularExpressionValidatorPhone.aspx</title></head> <body> <form Runat="Server"> Phone Number: <br> <asp:TextBox   id="txtPhone"   Columns="30"   Runat="Server"/> <asp:RegularExpressionValidator   ControlToValidate="txtPhone"   Display="Dynamic"   Text="Invalid Phone Number!"   ValidationExpression="\(?\s*\d{3}\s*[\)\.\-]?\s*\d{3}\s*[\-\.]?\s*\d{4}"   Runat="Server" /> <p> <asp:Button   Text="Submit"   OnClick="Button_Click"   Runat="Server"/> </form> </body> </html>

The C# version of this code can be found on the CD-ROM.

Validating Web Addresses

You also might need to validate URLs that users enter into a form at your Web site. For example, you might have a registration form that contains a field for the URL of a user's home page.

You can use the following regular expression to check for a valid URL:

http://\S+\.\S+

This regular expression matches any string that begins with the characters http:// followed by one or more nonwhitespace characters, followed by a period, followed by one or more nonwhitespace characters.

The page in Listing 3.8 demonstrates how you can use this regular expression with RegularExpressionValidator .

Listing 3.8 `RegularExpressionValidatorWeb.aspx`

 <Script Runat="Server"> Sub Button_Click( s As Object, e As EventArgs )   If IsValid Then     Response.Redirect( "thankyou.aspx" )   End If End Sub </Script> <html> <head><title>RegularExpressionValidatorWeb.aspx</title></head> <body> <form Runat="Server"> Enter the address of your homepage: <br> <asp:TextBox   id="txtHomepage"   Columns="50"   Runat="Server"/> <asp:RegularExpressionValidator   ControlToValidate="txtHomepage"   Display="Dynamic"   Text="Invalid URL!"   ValidationExpression="http://\S+\.\S+"   Runat="Server" /> <p> <asp:Button   Text="Submit"   OnClick="Button_Click"   Runat="Server"/> </form> </body> </html>

The C# version of this code can be found on the CD-ROM.

One problem with the code in Listing 3.8 concerns case-sensitivity . The RegularExpressionValidator control uses case-sensitive comparisons. So, the regular expression discussed in this section matches

http://www.superexpert.com

but does not match

HTTP://www.superexpert.com

Unfortunately, RegularExpressionValidator does not have a property that you can set to enable regular expression options such as the i option (an option which enables case-insensitive matches). You cannot use the i option inline because the syntax for doing so with JavaScript is different from the syntax for doing so with the .NET classes.

A not completely satisfactory workaround to this problem is illustrated by the page in Listing 3.9. The RegularExpressionValidator in this page performs a case insensitive match by using the i option. Furthermore, the RegularExpressionValidator disables client-side validation to prevent conflicts with JavaScript.

This is not a perfect workaround to the problem since the page must be submitted back to the server before the RegularExpressionValidator will validate the expression.

Listing 3.9 `RegularExpressionIgnoreCase.aspx`

 <Script Runat="Server"> Sub Button_Click( s As Object, e As EventArgs )   If IsValid Then     Response.Redirect( "thankyou.aspx" )   End If End Sub </Script> <html> <head><title>RegularExpressionValidatorIgnoreCase.aspx</title></head> <body> <form Runat="Server"> Enter the address of your homepage: <br> <asp:TextBox   id="txtHomepage"   Columns="50"   Runat="Server"/> <asp:RegularExpressionValidator   ControlToValidate="txtHomepage"   Display="Dynamic"   Text="Invalid URL!"   EnableClientScript="False"   ValidationExpression="(?i:http://\S+\.\S+)"   Runat="Server" /> <p> <asp:Button   Text="Submit"   OnClick="Button_Click"   Runat="Server"/> </form> </body> </html>

The C# version of this code can be found on the CD-ROM.

Checking for Entry Length

You also can use RegularExpressionValidator to check whether a form field contains more than a certain number of characters. This type of validation is especially useful when you're working with MultiLine TextBox controls. Because a MultiLine TextBox control does not have a MaxLength property, there is nothing to prevent a user from typing any number of characters in the text box.

To check for a certain entry length, use regular expression quantifiers like this:

 .{0,10}

This expression matches any entry (including spaces) that contains between 0 and 10 characters.

If you want to restrict the entry to a string of nonwhitespace characters that has a certain length, you would use a regular expression that looks like this:

 \S{0,10}

The page in Listing 3.10 demonstrates how you can use this regular expression with RegularExpressionValidator .

Listing 3.10 `RegularExpressionValidatorLength.aspx`

 <Script Runat="Server"> Sub Button_Click( s As Object, e As EventArgs )   If IsValid Then     Response.Redirect( "ThankYou.aspx" )   End If End Sub </Script> <html> <head><title>RegularExpressionValidatorLength.aspx</title></head> <body> <form Runat="Server"> Enter your last name: <br>(no more than 10 characters) <br> <asp:TextBox   id="txtLastname"   Columns="50"   Runat="Server"/> <asp:RegularExpressionValidator   ControlToValidate="txtLastname"   Display="Dynamic"   Text="Your last name can contain         a maximum of 10 characters and no spaces!"   ValidationExpression="\S{0,10}"   Runat="Server" /> <p> <asp:Button   Text="Submit"   OnClick="Button_Click"   Runat="Server"/> </form> </body> </html>

The C# version of this code can be found on the CD-ROM.

Validating ZIP Codes

You also can use the RegularExpressionValidator control to validate ZIP codes. For example, if you want to require that a ZIP code entered into a form field contains exactly five digits, you would use the following regular expression:

 \d{5}

This expression matches strings that have no more or no fewer than five digits.

The page in Listing 3.11 demonstrates how you would use this regular expression with the RegularExpressionValidator control.

Listing 3.11 `RegularExpressionValidatorZip.aspx`

 <Script Runat="Server"> Sub Button_Click( s As Object, e As EventArgs )   If IsValid Then     Response.Redirect( "ThankYou.aspx" )    End If End Sub </Script> <html> <head><title>RegularExpressionValidatorZip.aspx</title></head> <body> <form Runat="Server"> ZIP Code: <asp:TextBox   id="txtZipCode"   Columns="8"   Runat="Server"/> <asp:RegularExpressionValidator   ControlToValidate="txtZipCode"   Display="Dynamic"   Text="Invalid ZIP Code!"   ValidationExpression="\d{5}"   Runat="Server" /> <p> <asp:Button   Text="Submit"   OnClick="Button_Click"   Runat="Server"/> </form> </body> </html>

The C# version of this code can be found on the CD-ROM.

Validating Expressions: The RegularExpressionValidator Control