How to Use Regular Expressions to Constrain Input

Summary

This How to shows how you can use regular expressions within ASP.NET applications to constrain untrusted input. Regular expressions are a good way to validate text fields such as names, addresses, phone numbers, and other user information. You can use them to constrain input, apply formatting rules, and check lengths. To validate input captured with server controls, you can use the RegularExpressionValidator control. To validate other forms of input, such as query strings, cookies, and HTML control input, you can use the System.Text.RegularExpressions.Regex class.

This How-to shows how you can use regular expressions within ASP.NET applications to constrain untrusted input.

Overview

If you make unfounded assumptions about the type, length, format, or range of input, your application is unlikely to be robust. Input validation can become a security issue if an attacker discovers that you have made unfounded assumptions. The attacker can then supply carefully crafted input that compromises your application by attempting SQL injection, cross-site scripting, and other injection attacks. To avoid such vulnerability, you should validate text fields (such as names, addresses, tax identification numbers, and so on) and use regular expressions to do the following:

• Constrain the acceptable range of input characters.
• Apply formatting rules. For example, pattern-based fields, such as tax identification numbers, ZIP Codes, or postal codes, require specific patterns of input characters.
• Check lengths.

Regular expression support is available to ASP.NET applications through the RegularExpressionValidator control and the Regex class in the System.Text.RegularExpressions namespace.

Using a RegularExpressionValidator Control

If you capture input by using server controls, you can use the RegularExpressionValidator control to validate that input. You can use regular expressions to restrict the range of valid characters, to strip unwanted characters, and to perform length and format checks. You can constrain the input format by defining patterns that the input must match.

To validate a server control's input using a RegularExpressionValidator

1. Add a RegularExpressionValidator control to your page.
2. Set the ControlToValidate property to indicate which control to validate.
3. Set the ValidationExpression property to an appropriate regular expression.
4. Set the ErrorMessage property to define the message to display if the validation fails.

The following example shows a RegularExpressionValidator control used to validate a name field.

<%@ language="C#" %>
<form id="form1" runat="server">
    <asp:TextBox ID="txtName" runat="server"/>
    <asp:Button ID="btnSubmit" runat="server" Text="Submit" />
    <asp:RegularExpressionValidator ID="regexpName" runat="server"     
                                    ErrorMessage="This expression does not validate." 
                                    ControlToValidate="txtName"     
                                    ValidationExpression="^[a-zA-Z'.\s]{1,40}$" />
</form>  

The regular expression used in the preceding code example constrains an input name field to alphabetic characters (lowercase and uppercase), space characters, the single quotation mark (or apostrophe) for names such as O'Dell, and the period or dot character. In addition, the field length is constrained to 40 characters.

Using ^ and $

Enclosing the expression in the caret (^) and dollar sign ($)markers ensures that the expression consists of the desired content and nothing else. A ^ matches the position at the beginning of the input string and a $ matches the position at the end of the input string. If you omit these markers, an attacker could affix malicious input to the beginning or end of valid content and bypass your filter.

Using the Regex Class

If you are not using server controls (which means you cannot use the validation controls) or if you need to validate input from sources other than form fields, such as query string parameters or cookies, you can use the Regex class within the System.Text.RegularExpressions namespace.

To use the Regex class

1. Add a using statement to reference the System.Text.RegularExpressions namespace.
2. Call the IsMatch method of the Regexclass, as shown in the following example.

   // Instance method:
   Regex reg = new Regex(@"^[a-zA-Z'.]{1,40}$");
   Response.Write(reg.IsMatch(txtName.Text));
   // Static method:
   if (!Regex.IsMatch(txtName.Text, 
                      @"^[a-zA-Z'.]{1,40}$"))
   {
     // Name does not match schema
   }  

For performance reasons, you should use the static IsMatch method where possible to avoid unnecessary object creation.

The following example shows how to use a regular expression to validate a name input through a regular client-side HTML control.

<%@ Page Language="C#" %>

<html  >
  <body>
    <form id="form1" method="post" action="HtmlControls.aspx">
        Name:
        <input name="txtName" type="text" />
        <input name="submitBtn" type="Submit" value="Submit"/>
    </form>
  </body>
</html>

<script runat="server">

  void Page_Load(object sender, EventArgs e)
  {
    if (Request.RequestType == "POST")
    {
      string name = Request.Form["txtName"];
      if (name.Length > 0)
      {
        if (System.Text.RegularExpressions.Regex.IsMatch(name, 
                                           "^[a-zA-Z'.]{1,40}$"))
          Response.Write("Valid name");
        else
          Response.Write("Invalid name");
      }
    }
  }

</script>  

Use Regular Expression Comments

Regular expressions are much easier to understand if you use the following syntax and comment each component of the expression by using a number sign (#). To enable comments, you must also specify RegexOptions.IgnorePatternWhitespace, which means that non-escaped white space is ignored.

Regex regex = new Regex(@"
                        ^           # anchor at the start
                       (?=.*\d)     # must contain at least one numeric character
                       (?=.*[a-z])  # must contain one lowercase character
                       (?=.*[A-Z])  # must contain one uppercase character
                       .{8,10}      # From 8 to 10 characters in length
                       \s           # allows a space 
                       $            # anchor at the end", 
                       RegexOptions.IgnorePatternWhitespace);  

Common Regular Expressions

Some common regular expressions are shown in Table 1.

Table 1. Common Regular Expressions

Adapted from Microsoft patterns & practices guidance.