Ask Ben: Getting The Domain Name From The Referer URL

Hi Ben Nadel, I want to use regex code to extract only domain name for http referrers, can you please give me clue? thanks.

Normally, when we think about the domain name of a URL (which is what the CGI.http_referer value is), we think of the domain name as the part of the URL that comes after the protocol (http://, https://, ftp://, etc.). As such, we could grab the domain name by using a positive look behind on the protocol. However, the regular expression engine that ColdFusion uses (POSIX) does not allow for look behinds. As such, we can either reach down into the Java layer and user the Java Pattern library; or, we can hack the POSIX engine to do what we need.

Since reaching down into the Java layer is probably overkill for such a use case, I will instead explore two different methods to use the POSIX regex engine to get what we need. One method will use the REReplace() function and one method will use the REMatch() function (only available in ColdFusion 8 or later).

Using REReplace() To Get The Domain Name

While it might not seem intuitive to use a replace function to extract part of a string, if we replace the entire string with the substring that we are seeking, what do we end up with? Our target substring. Because the REReplace() function allows use to capture groups in our regular expression and then use them within our "replacement text," we have the ability to replace our original string with just our target string:

Notice that in the regular expression pattern, we are matching the entire URL, but we are only capturing the domain value. In doing so, it allows us to reference the captured domain in our replacement text. And, as you can see, we are replacing the entire URL with the value of the captured group - our domain. And, when we run this code, we get the following output:

Referer Domain: www.shemuscle.com

Using REMatch() To Get The Domain Name

If you are using ColdFusion 8, you can use the REMatch() function to gather all matches in a given string. We can use this function to match parts of the target URL and then pluck the domain name out of the returned matches. Because regular expressions are evaluated from left to right in a greedy fashion, we can have our regular expression pattern match parts of the domain moving from left to right; first, we'll match the protocol, then the domain name, then the rest of the string:

Notice in this code that our regular expression matches the three crucial parts of the domain. And, when we run this code, we get the following output:

Part: http://
Part: www.shemuscle.com
Part: /category/anonymous/

In this case, the domain name is the second item matched and can be extracted from the matches using urlParts[ 2 ].

Using java.net.URL To Get The Domain Name

As a final method, let's quickly explore the Java URL object. In the previous examples, we had to do all of the heavy lifting ourselves in terms of figuring out how to parse the URL using regular expressions. Well, if we use the Java class, java.net.URL, we can offload that heavy lifting. If we create an instance of the Java URL class and initialize it with our target URL, it will parse the URL internally and give use access to the URL components:

Notice that all we have to do is create the URL instance and pass in our referer URL. The Java URL takes care of the rest. Then, all we have to do is ask it of for the domain name (host) of the given URL. And, when we run the above code, we get the following output:

Referer Domain: www.shemuscle.com

Works like a charm and we didn't have to get our hands dirty with any regular expressions.

Regular expression are a great tool in the programming toolbox; and, they are amazing for string parsing. But sometimes, we can offload the processing of strings to existing pieces of functionality like the Java URL class, and get what we need without any of the complexity associated with regular expressions. I hope this helps!

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