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Learn how to write simple regular expressions with optional characters.
Practice
Copy each set of test strings into a Regex tester like regexpal or 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'd most likely glance at them both and
say, hm, yes.
0:03
However, a regular expression
doesn't work that way.
0:08
Instead, it uses a parser which
compares each character in a regular
0:11
expression with a character in
the string in the same position.
0:16
In other words,
the regex parser requires two things.
0:20
An expression, also called a pattern,
and a string to match.
0:23
The parser takes that pattern and
walks through the string,
0:28
character by character.
0:31
First, it checks if there is a t.
0:33
Then if the t is followed by an o.
0:36
Which is followed by a y, then a b, o,
0:39
a, all the way to the t at the end.
0:43
At that point,
the parser knows the string is a match.
0:47
I'm going to type toyboat into regexpal,
in the regex pane, in the testing pane.
0:51
You can see there is
a match selected here.
0:58
This is the basic matching we saw earlier.
1:01
Let's say we want to match the singular or
the plural.
1:04
If I add an s to the regex,
we lose our first match.
1:09
However, we can make
this final s optional.
1:14
Match it if it exists but
don't require it.
1:18
We can do that with another special
regex character, the question mark.
1:21
The question mark matches the character
that appears directly before it
1:25
zero times or one time.
1:29
In this case, the lower case s is before
the question mark, so s is optional.
1:32
But if it does appear,
it can only appear once.
1:38
So this regex will match toyboat or
toyboats but not toyboatss with two S's.
1:41
Let's use the question mark
in the middle of the string.
1:48
Let's match toybots, too.
1:50
We can put a question mark after
the a to make it optional also.
1:53
It works.
1:57
I'll replace toybots with toyboats again,
but this time with a space.
2:00
Just like before,
2:05
we can handle this with a question mark,
making the space optional.
2:06
Now let's say that we want to match toy,
even if it's capitalized.
2:14
Regular expressions are case
sensitive by default, so
2:20
we'll need to specify that the T
can be upper or lower case.
2:23
We can tell the interpreter
to accept a capital or
2:28
lower case t by wrapping both
characters in square brackets.
2:31
Let me show you what I mean.
2:36
I'll put a capital T next to the lowercase
t, and surround both with square brackets.
2:38
I just added what is called
a character set to the regex.
2:46
When the interpreter sees it,
2:50
exactly one of the characters in the set
will match one position in the string.
2:51
We can specify as many characters
as we want between these brackets.
2:57
Let's say we also want to match joy
boats with a lowercase j, for example.
3:01
We can just put a j inside
the square brackets.
3:06
There's two things to note.
3:10
No matter how many characters
we put in the set,
3:11
the set will always match only
one position of the string.
3:14
In this case, it's the first position.
3:18
It also makes no difference how
you order the character set.
3:21
Now I wanted to give you a challenge.
3:24
I'll type out two more test cases for you.
3:26
See if you can modify
the regex to match those
3:29
along with the strings it already matches.
3:32
I'll add a hyphenated version.
3:35
And another version
with the B capitalized.
3:44
Go ahead and pause the video and
see if you can do it.
3:48
For the hyphenated string,
I replaced the space and
3:52
the regex with a character set
containing a space and a hyphen.
3:55
Note that the question mark now indicates
that the character set is optional.
4:02
In other words, the character between
the y and the b can either be a space,
4:08
a hyphen, or nothing at all.
4:13
For the second string,
4:16
I used a character set to match
either a lower or upper case B.
4:17
I've added some additional strings for
4:24
you to practice on in
the teachers notes below.
4:26
Take some time to try
then now if you'd like.
4:28
In the next video, I'll show you how to
add a little more power to character sets.
4:32
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