 Let's take a look at another common example, URLs. Here is a regular expression for URLs I came up with earlier. It isn't perfect, perhaps by the end of this video you'll have figured out some things that are missing, but it's a good starting example, so let's take a look. Before we do though, we need to make one more clarification. Apart from some cases, such as backslash D, the backslash is used as an escape character, and so the following symbol is matched as is. We found before that the period matches any single character, but backslash period matches the period itself. This is the same for backslash forward slash, as forward slash normally matches the division symbol. So, first we have the hypertext transfer protocol, HTTP, with 0 or 1 of an S, followed by a colon forward slash forward slash. This is the part of the URL you see before the actual web address. This entire sequence can be left out, so S is enclosed in parentheses and followed by a question mark. The next sequence, the main web address, is a combination of characters and symbols. We have a choice between a letter or a digit, or a period, or a hyphen, or a forward slash, but we almost certainly need more than one, so it is wrapped in parentheses and given a plus symbol. After this, there is a period, then the suffix. .com is the most common example, but there are many regional alternatives, such as .co.nz for New Zealand and .co.uk for the United Kingdom. In this series of videos, we took a look at the basic meaning of regular expressions, what they're used for as well as some of the many different symbols used in practical systems, though they can all be expressed in terms of just four. For more information and examples, check out the Computer Science Field Guide. Regular expressions is in the chapter on formal languages. Thanks for watching.