Validate a URL using regular expressions

Today, I had to build web form that took user input from standard ASP.NET input controls. In one of the text boxes the user must to enter a valid URL, so I had to make some validation logic. But first of all, I had to find out what kind of URL’s we would accept as being valid. These are the rules we decided upon:

• The protocol must be http or https
• Sub domains are allowed
• Query strings are allowed

Based on those rules, I wrote this regular expression:

(http|https)://([\w-]+\.)+[\w-]+(/[\w- ./?%&=]*)?

It is used in a RegularExpressionValidator control on the web form and on a business object in C#.

<asp:RegularExpressionValidator runat="Server"

  ControlToValidate="txtUrl"

  ValidationExpression="(http|https)://([\w-]+\.)+[\w-]+(/[\w- ./?%&=]*)?"

  ErrorMessage="Please enter a valid URL"

  Display="Dynamic" />

Here is the server-side validator method used by the business object:

using System.Text.RegularExpressions;

private bool IsUrlValid(string url)

{

  return Regex.IsMatch(url, @"(http|https)://([\w-]+\.)+[\w-]+(/[\w- ./?%&=]*)?");

}

You can add more protocols to the expression easily. Just add them to the beginning of the regular expression:

(http|https|ftp|gopher)://([\w-]+\.)+[\w-]+(/[\w- ./?%&=]*)?

You can also allow every thinkable protocol containing at least 3 characters by doing this:

([a-zA-Z]{3,})://([\w-]+\.)+[\w-]+(/[\w- ./?%&=]*)?