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Learn how to write simple regular expressions with optional characters.
Practice
Copy each set of test strings into regex101. Using what you've learned so far, create a regular expression that will match all of the strings in the set.
1 )
ladybug
ladybugs
lady bugs
2 )
ladybug
lady bugs
lazy bug
lazy lug
3 )
ladybug
lazy lug
lazy slug
hazy slug
4 )
ladybug
fading rug!
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If I ask you whether
these two words match,
0:00
you'll most likely glance at them both and
say yes.
0:03
However, a regular expression
doesn't work that way.
0:06
Instead, it uses a parser, which compares
each character in a regular expression
0:09
with the character in the string
in the same position.
0:14
In other words,
the RegeEx parser requires two things,
0:18
an expression, also called a pattern,
and a string to match.
0:22
The parser takes that pattern and
0:27
walks through the string,
character by character.
0:29
First it checks if there is a t, then
if the t is followed by an o and so on.
0:33
[SOUND] It's a match.
0:39
I'm going to type the word toyboat
into RegEx 101 in the RegEx pane,
0:45
and then the testing pane.
0:50
You can see that there's
a match selected here.
0:54
This is the basic match we saw earlier.
0:58
Let's say we want to match the singular or
the plural version.
1:01
If I add an s in the RegEx,
we lose our first match.
1:08
However, we can make
this final s optional,
1:14
match it if it exists,
but don't require it.
1:19
We can do that with another special
RegEx character, the question mark.
1:22
The question mark matches the character
that appears directly before it,
1:27
zero or one time.
1:33
In this case, the lower case s
is before the question mark.
1:35
So s is optional.
1:40
But if it does appear,
it can only appear once.
1:43
So this RegEx will match toyboat or
toyboats,
1:46
but not toyboats with two s.
1:51
Let's use a question mark in
the middle of the string.
1:55
Let's match toybots as well.
1:58
We can put a question mark after
the a to make it optional also.
2:06
It works.
2:11
I'll replace toybots with toyboats again.
2:13
But I'm also going to add
one with a space, toy boats.
2:18
Just like before,
we can handle this with a question mark.
2:23
Now let's say we want to match toy,
even if it's capitalized.
2:31
Toy boats.
2:39
Regular expressions are case
sensitive by default, so
2:42
we'll need to specify that the T
can be uppercase or lowercase.
2:45
We can tell the interpreter
to accept a capital or
2:51
lowercase T by wrapping both
characters in square brackets.
2:54
Let me show you what I mean.
2:59
I'll put a capital T next
to the lowercase t, and
3:02
I'll surround them both
in square brackets.
3:06
I just added what is called
a character set to the RegEx.
3:09
When the interpreter sees it,
3:14
exactly one of the characters in the set
will match one position in the string.
3:16
We can specify as many characters
as we want between these brackets.
3:21
Let's say we also want to match joy boats,
with a lowercase j.
3:26
I'll add joy boats.
3:34
There's two things to note.
3:38
No matter how many characters
we put in the set,
3:39
the set will match only one
position of the string.
3:42
In this case, it's the first position.
3:46
It also makes no difference how
you order the character set.
3:48
Now I want to give you a challenge.
3:53
I'll type out two more test cases for you.
3:55
See if you can modify the RegEx
to match those as well.
3:58
I'll add a hyphenated version, toy-boats,
4:02
and I'll also add one with a capital B.
4:08
Go ahead and pause this video and
see if you can do it.
4:14
For the hyphenated string,
I'll replace the space in the RegEx
4:18
with a character set containing
both a space and a hyphen.
4:23
Note that the question mark now indicates
that the character set is optional,
4:30
in other words,
the character between the y and
4:35
the b can either be a space,
a hyphen, or nothing at all.
4:39
For the second string,
4:45
I use a character set to match
either a lower or upper case B.
4:47
I've added some additional strings for
you to practice in the teacher's notes.
4:55
Take some time now to give them a try.
5:00
In the next video, I'll show you how to
add a little more power to character sets.
5:03
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