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That's about it for regular expressions. It seems very difficult, but once you grasp it is extremely easy yet one of
most powerful tools when programming in PHP. I can't count number of times regex has saved me from hours of coding difficult text functions.
An Example What would a good tutorial be without some real examples? Let's first have a look at a simple e-mail validation function. An e-mail address must start with letters or numbers, then have a @, then a domain, ending with an extension. The regex for that would be something like this: ^[a-zA-Z0-9_.-]+@[a-zA-Z0-9-]+.[a-zA-Z0-9-.]+$
Let me quickly explain that regex. Basically, first part says that it must all be letters or numbers. Then we get @, and after that there should be letters and/or numbers again (the domain). Finally we check for a period, and then for an extension. The code to use this regex looks like this:
<?php// Good e-mail $good = "john@example.com";
// Bad e-mail $bad = "blabla@blabla";
// Let's check good e-mail if (preg_match("/^[a-zA-Z0-9_.-]+@[a-zA-Z0-9-]+.[a-zA-Z0-9-.]+$/", $good)) { echo "Valid e-mail"; } else { echo "Invalid e-mail"; }
echo '<br />';
// And check bad e-mail if (preg_match("/^[a-zA-Z0-9_.-]+@[a-zA-Z0-9-]+.[a-zA-Z0-9-.]+$/", $bad)) { echo "Valid e-mail"; } else { echo "Invalid e-mail"; }
?>
The result of this would be "Valid E-mail. Invalid E-mail", of course. We have just checked if an e-mail address is valid. If you wrap above code in a function, you've got yourself a e-mail validation function. Keep in mind though that regex isn't perfect: after all, it doesn't check whether extension is too long, does it? Because I want to keep this tutorial short, I won't give full fledged regex, but you can find it easily via Google.Another Example Another great example would be a telephone number. Say you want to verify telephone numbers and make sure they were in correct format. Let's assume you want numbers to be in format of xxx-xxxxxxx. The code would look something like this:
<?php// Good number $good = "123-4567890";
// Bad number $bad = "45-3423423";
// Let's check good number if (preg_match("/d{3}-d{7}/", $good)) { echo "Valid number"; } else { echo "Invalid number"; }
echo '<br />';
// And check bad number if (preg_match("/d{3}-d{7}/", $bad)) { echo "Valid number"; } else { echo "Invalid number"; }
?>
The regex is fairly simple, because we use d. This basically means "match any digit" with length behind it. In this example it first looks for 3 digits, then a '-' (hyphen) and finally 7 digits. Works perfectly, and does exactly what we want.What exactly is possible with Regular Expressions? Regular expressions are actually one of most powerful tools in PHP, or any other language for that matter (you can use it in your mod_rewrite rules as well!). There is so much you can do with regex, and we've only scratched surface in this tutorial with some very basic examples.
If you really want to dig into regex I suggest you search on Google for more tutorials, and try to learn regex syntax. It isn't easy, and there's quite a steep learning curve (in my opinion), but best way to learn is to go through a lot of examples, and try to translate them in plain English. It really helps you learn syntax.
In future I will dedicate a complete article to strictly examples, including more advanced ones, without any explanation. But for now, I can only give you links to other tutorials: The 30 Minute Regex Tutorial Regular-Expressions.info
Dennis Pallett is a young tech writer, with much experience in ASP, PHP and other web technologies. He enjoys writing, and has written several articles and tutorials. To find more of his work, look at his websites at http://www.phpit.net, http://www.aspit.net and http://www.webdev-articles.com
