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8.5 Regular Expressions

Often a simple value or range check is insufficient; you must check that the form of the data entered is correct. For example, you may need to ensure that a ZIP code is five digits, an email address is in the form name@place.com, a credit card matches the right format, and so forth.

A regular expression validator allows you to validate that a text field matches a regular expression. Regular expressions are a language for describing and manipulating text. For more complete coverage of this topic, please see Mastering Regular Mastering Regular Expressions, Second Edition, by Jeffrey Friedl (O'Reilly).

A regular expression consists of two types of characters: literals and metacharacters . A literal is just a character you wish to match in the target string. A metacharacter is a special symbol that acts as a command to the regular expression parser. The parser is the engine responsible for understanding the regular expression. Consider this regular expression:

^\d{5}$

This will match any string that has exactly five numerals. The initial metacharacter, ^, indicates the beginning of the string. The second metacharacter, \d, indicates a digit. The third metacharacter, {5}, indicates exactly 5 of the digits, and the final metacharacter, $, indicates the end of the string. Thus, this regular expression matches five digits between the beginning and end of the line, and nothing else.

A slightly more sophisticated algorithm might accept either a 5-digit ZIP code or a 9 digit (plus 4) ZIP code in the format of 12345-1234. Rather than using the \d metacharacter, you could simply designate the range of acceptable values:

ValidationExpression="[0-9]{5}|[0-9]{5}-[0-9]{4}"

You create a RegularExpressionValidator much as you did the previous validators. The only new attribute is ValidationExpression, which takes a valid regular expression within quotation marks. For example, the following code fragment defines a regular expression validator to ensure that the value entered into a text box is a five-digit numeric ZIP code:

<asp:RegularExpressionValidator ID="regExVal"
 ControlToValidate="txtZip" Runat="server"
 ValidationExpression="^\d{5}$"
 display="Static">Please enter a valid 5 digit Zip code</asp:RegularExpressionValidator>

If the control pointed to by ControlToValidate has a string that matches the regular expression, validation succeeds. The complete .aspx source is shown in Example 8-6.

Example 8-6. Regular expression validator

<%@ Page language="c#" Codebehind="WebForm1.aspx.cs" AutoEventWireup="false" 
Inherits="RegularExpressionValidation.WebForm1" %>

<HTML>
  <HEAD>
<meta content="Internet Explorer 5.0" name=vs_targetSchema>
<meta content="Microsoft Visual Studio 7.0" name=GENERATOR>
<meta content=C# name=CODE_LANGUAGE>
  </HEAD>
<body MS_POSITIONING="GridLayout">
<form method=post runat="server">
    <table>
      <tr>
         <td colspan="2">
            <h5>Please enter your Zip Code</h5>         
         </td>
      </tr>
      <tr>
         <td>
            <asp:TextBox width="60" ID="txtZip" runat="server" />
         </td>
         <td>
            <asp:RegularExpressionValidator ID="regExVal"
             ControlToValidate="txtZip" Runat="server"
             ValidationExpression="^\d{5}$"
             display="Static">Please enter a valid 5 digit Zip 
             code</asp:RegularExpressionValidator>
         </td>
       </tr>
       <tr>
         <td>
            <asp:Button ID="btnValidate" Text="Validate" 
              Runat="server"></asp:Button>
         </td>
         <td>
            <asp:Label ID="lblMsg" Runat="server" Text=""/>
         </td>
       </tr>
    </table></FORM>
  </body>
</HTML>

When you use a RegularExpressionValidator control with client-side validation, the regular expressions are matched using JScript. This may differ in small details from the regular expression checking done on the server.