Question

I know a little about preg_match, however there are some that look rather complex and some that contain symbols that I don’t entirely understand. For example:

On the first one – I can only assume this has something to do with an e-mail address and url, but what do things like [^/] and the ? mean?

preg_match('@^(?:http://)?([^/]+)@i', $variable);

…..

In the second one – what do things like the ^, {5} and $ mean?

preg_match("/^[A-Z]{5}[0-9]{4}[A-Z]{1}$/", $variable);

It’s just these small things I’m not entirely sure on and a brief explanation would be much appreciated.

Answer 1

Here are the direct answers. I kept them short because they won’t make sense without an understanding of regex. That understanding is best gained at http://www.regular-expressions.info/tools.html. I advise you to also try out the regex helper tools listed there, they allow you to experiment – see live capturing/matching as you edit the pattern, very helpful.

Simple parentheses ( ) around something makes it a group. Here you have (?=) which is an assertion, specifically a positive look ahead assertion. All it does is check whether what’s inside actually exists forward from the current cursor position in the haystack. Still with me?
Example: foo(?=bar) matches foo only if followed by bar. bar is never matched, only foo is returned.

With this in mind, let’s dissect your regex:

 /^.*(?=.{4,})(?=.*[0-9])(?=.*[a-z])(?=.*[A-Z]).*$/

Reads as:
    ^.* From Start, capture 0-many of any character
(?=.{4,}) if there are at least 4 of anything following this
(?=.*[0-9]) if there is: 0-many of any, ending with an integer following
(?=.*[a-z]) if there is: 0-many of any, ending with a lowercase letter following
(?=.*[A-Z]) if there is: 0-many of any, ending with an uppercase letter following
    .*$ 0-many of anything preceding the End