Validate With Regular Expressions (Continued)

Validate a Domain Address
The example I've just shown you demonstrates the most common constructs you use when you write regular expressions to validate user input. Here's another simple example that shows how you can use other input characters to validate input (see Listing 2). Suppose you want to parse a domain address. The addresses fawcette.com, microsoft.com, and srtsolutions.com are all valid. However, any address that uses a different protocol (such as ftp://) or contains invalid characters is invalid. A valid domain name must have fewer than 64 characters, which can include only a–z, 0–9, and -. You must have no more than 63 characters, no more than one period (.), and a suffix in order to validate a domain name. I'll limit the list of valid suffixes to .com, .net, .org, .edu, .gov, and .mil, to keep this example reasonably simple.

Once again, you build the regular expression by using ^ and $ for the start and end of the entire input string. You need to find between 1 and 63 characters in the set of a–z, 0–9, and -. You place the set of valid character ranges inside the brackets. This set includes the range a–z, the range 0–9, and the single hyphen character. The {1,63} construct matches from 1 to 63 repeats of the preceding range. This expression builds on a construct you saw previously:

[a-z,0-9,-]{1,63}

Next, you must find a single period. This might seem simple, but a period is a special character in the regular expression language, so you need to escape the character by preceding it with a backslash (\). Finally, you must find one of the approved extensions. You find one of a set of phrases by placing all the phrases between parentheses, separated by the pipe character (|): (com|net|org|edu|gov|mil). The complete expression is:

^[a-z,0-9,-]{1,63}\\.
(com|org|net|gov|mil)$

Note that you need only two lines of code for each expression in the examples I've shown you. Putting together regular expressions can take work, but it pays off handsomely. I've yet to see an input format that you can't validate with regular expressions. Look at the samples provided in the ASP.NET regular expression validator to learn more about using and forming your own regular expressions. Try to understand how each one works. Then, build expressions for your own validations. Test all the small subexpressions individually and include comments in your code, because debugging a long expression can be tricky.