uccser/cs-field-guide

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Computer Science Education Research,
University of Canterbury, New Zealand
Subtitle file for the video "Regular Expressions - 2 - Basic Symbols"
Author: Alasdair Smith
Language: English
Date: 20/06/17

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Let’s take a look at how we use
the four basic symbols in a Regular Expression,

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starting with the parentheses.

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In arithmetic, if we have the equation 1 + 2 x 3,
we follow the order of arithmetic expressions

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and solve the multiplication first.
So we get 1 + 6 which equals 7,

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but if we add parentheses around 1 + 2,
it holds the highest precedence and so we get 3 x 3,

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which equals 9. In regex, parentheses work
in the same way, specifying the highest precedence.

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Next we have the Kleene star, named after the man
who helped found the concept of Regular Expressions.

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This matches the preceding symbol
repeated zero or more times.

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For example, BA* will match
B and BA and BAA and BAAA and so on.

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Lastly we have the alternation symbol.
This can be read as a choice,

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so the expression A(A|B)
matches both AA and AB.

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We usually refer to this as ‘bar’ and read it as ‘or.’
<01:25.400>Bar has the lowest precedence of the basic regex symbols,

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so A|B* is the same as saying A|(B*),
which means you can either match an A, or any number of Bs.