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A Regular Expression lets you match patterns in strings. We'll start with a simple example, using the Chrome browser's find functionality.
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[MUSIC]
0:00
Hi, I'm Alena, a developer and
teacher at Treehouse.
0:09
Welcome to your new superpower.
0:13
Regular expressions are used everywhere.
0:16
Not only in every programming
language you'll ever use, but
0:18
also in most computer applications,
text editors,
0:22
word processors, spreadsheets and
even graphics editors.
0:26
Regular expressions are like
having a secret code for
0:31
telling a computer exactly what you want.
0:34
Regex, as it's often referred to,
is a text string for
0:38
describing a search pattern.
0:42
You can think of it as a supercharged
version of a fine tool.
0:45
In this course, I'll show you
what regular expressions are, and
0:49
we'll get some practice using them.
0:54
At the end, I'll provide resources for
language specific implementations.
0:56
Let's start by looking at a simple
example using a browser's find feature.
1:01
I'm on the Wikipedia page for
regular expressions.
1:07
It says here, that Stephen Cole Kleene,
1:11
formalized a description
of a regular language.
1:13
Let's find his name on the page.
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I'll hit Cmd+F to open Chrome's find bar.
1:19
As I type Kleene's last name,
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notice that chrome highlights
each place the word appears.
1:26
These highlighted strings are called
matches because they match the string
1:30
I'm typing in.
1:35
The matches here are case insensitive.
1:37
In other words,
even though I didn't type a capital K,
1:40
Chrome still matches the word Kleene.
1:44
If I then type a space,
notice that this Kleene is no longer
1:47
highlighted because it no longer matches
the search that I'm looking for.
1:52
If I backspace and type an apostrophe,
this one is now selected,
1:58
but the one up here is no longer selected.
2:03
What happens if I want to
select both of these at once?
2:06
You can do that using a special
character in a regular expression.
2:10
Because Chrome's simple search bar
doesn't recognize regular expressions,
2:14
let's use an online
editor called Regex101.
2:18
Regex is a common way to shorten the term,
regular expressions.
2:24
On this page, we can test regex and
interactively learn how they work.
2:28
You can see some extra
options on the left here.
2:34
We're not gonna be using any of them,
so I can zoom in a bit.
2:36
The pane on the right
will be extra helpful,
2:41
as you step through your
own regular expressions.
2:44
So before I zoom in too far,
let me show you how these work.
2:46
Type the text you want to
match in the lower window.
2:51
This is kind of like the Wikipedia page.
2:54
I'll type Kleene's name.
2:57
In the upper text field,
I'll also type Kleene.
2:59
The top's text field is like
the find bar of the browser.
3:05
In the bottom field,
Kleene highlights, it's a match.
3:09
The explanation pane on the top
shows us that our regular
3:13
expression matches the characters
literally and is case-sensitive.
3:19
This is a string match, a kind of regex
that only matches exact duplicates.
3:25
Next, you'll notice
the global pattern flag.
3:31
If we look at the left to
see the regular expression,
3:34
we see that it ends with a g and an m.
3:37
For all our work in this course, we're
going to leave these default flags set.
3:40
The first flag is a g,
which stands for global.
3:45
This configures the interpreter
to return all matches,
3:51
not stop after the first match.
3:55
The second flag is an m,
which stands for multi line.
3:57
This configures the interpreter to treat
each new line as a separate test string,
4:03
as opposed to one big block of text.
4:08
After that, you'll see a pane
with the details about the match.
4:11
And finally,
a quick reference guide below.
4:16
I'm going to zoom in a little more,
so that it's easier for
4:21
you to follow what I'm doing.
4:24
But remember, those panes are there to
help you while you're building your
4:26
own regular expressions.
4:29
You type the regular expression
into the top field, and
4:31
you type any text you want to
search in the bottom field.
4:35
In the bottom field,
I'll type Kleene again.
4:39
And you see that both
instances are matched.
4:43
Let's replicate the search we did
earlier on the Wikipedia page.
4:46
I'll put a space after one and
an apostrophe after the other.
4:50
Now when I type a space in the upper
field, we see a similar behavior.
4:56
Only one of them is matched.
5:01
When I type an apostrophe instead,
the other is matched.
5:03
So far, I've just been putting
strings into the top field, but
5:08
we can use other special characters to
create a more flexible regular expression.
5:13
For example, I'll add a dot or
period to the end of this, and
5:19
you can see that both strings are matched.
5:24
A dot has a special meaning
in a regular expression.
5:26
It matches any single character, so
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it will match both the space and
the apostrophe.
5:33
I discuss this and other special regex
characters in more detail later, so
5:37
don't worry too much about it for now.
5:41
I just wanted to give you
a glimpse of what's possible.
5:44
In the next video, I'll teach you more
about special regex characters and
5:47
how you can use them to
find patterns in text.
5:51
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