uccser/cs-field-guide

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Computer Science Education Research,
University of Canterbury, New Zealand
Subtitle file for the video "Regular Expressions - 5 - A Bigger Example"
Author: Alasdair Smith
Language: English
Date: 20/06/17

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Let’s take a look at another common example, URLs.
<00:05.800>Here is a regular expression for URLs I came up with earlier.

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It isn’t perfect, perhaps by the end of this video
you’ll have figured out some things that are missing,

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but it’s a good starting example so let’s take a look.

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Before we do though we need to make
one more clarification.

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Apart from some cases such as backslash d,
the backslash is used as an escape character

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and so the following symbol is matched ‘as-is.’

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We found before that the period matches any single character,
but backslash period matches the period itself.

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This is the same for backslash forwardslash,
as forwardslash normally matches the division symbol.

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So, first we have the Hypertext Transfer Protocol, HTTP,
with zero or one of an S, followed by colon, forwardslash forwardslash.

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This is the part of the URL you see before the actual web address.
This entire sequence can be left out,

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so it is enclosed in parentheses
and followed by a question mark.

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The next sequence, the main web address,
is a combination of characters and symbols.

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We have a choice between a letter or a digit
or a period or a hyphen or a forwardslash,

NOTE the hyphen was accidentally left out in the drawing

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but we almost certainly need more than one,
so it is wrapped in parentheses and given a plus symbol.

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After this there is a period, then the suffix.
.com is the most common example in business,

NOTE 'in business' was accidentally left out in the audio

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but there are many many regional alternatives;
such as .co.nz for New Zealand and .co.uk for the United Kingdom.

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In this series of videos we took a look at
the basic meaning of Regular Expressions,

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what they’re used for, as well as some of
the many different symbols used in practical systems,

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though they all can be expressed in terms of just four.

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For more information and examples,
check out the Computer Science Field Guide.

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Regular Expressions is in the chapter on Formal Languages.

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Thanks for watching!