- What is a Regular Expression? 5:54
- Matching Specific Characters 5:09
- Matching Character Ranges 2:33
- Using Wildcard Characters 4:49
- Finding Repeated Characters 5:47
- Excluding Characters 3:53
- Alternation 1:40
- Groups 3:27
- Beginning and Ending of Strings 2:10
- Language Practice 1:03
- Regular Expressions 5 questions
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Matching Specific Characters
5:09Learn how to write simple regular expressions with optional characters.
If I ask you whether
these two words match,
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you'll most likely glance at them both and
say yes.
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However, a regular expression
doesn't work that way.
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Instead, it uses a parser, which compares
each character in a regular expression
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with the character in the string
in the same position.
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In other words,
the RegeEx parser requires two things,
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an expression, also called a pattern,
and a string to match.
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The parser takes that pattern and
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walks through the string,
character by character.
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First it checks if there is a t, then
if the t is followed by an o and so on.
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[SOUND] It's a match.
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I'm going to type the word toyboat
into RegEx 101 in the RegEx pane,
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and then the testing pane.
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You can see that there's
a match selected here.
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This is the basic match we saw earlier.
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Let's say we want to match the singular or
the plural version.
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If I add an s in the RegEx,
we lose our first match.
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However, we can make
this final s optional,
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match it if it exists,
but don't require it.
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We can do that with another special
RegEx character, the question mark.
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The question mark matches the character
that appears directly before it,
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zero or one time.
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In this case, the lower case s
is before the question mark.
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So s is optional.
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But if it does appear,
it can only appear once.
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So this RegEx will match toyboat or
toyboats,
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but not toyboats with two s.
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Let's use a question mark in
the middle of the string.
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Let's match toybots as well.
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We can put a question mark after
the a to make it optional also.
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It works.
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I'll replace toybots with toyboats again.
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But I'm also going to add
one with a space, toy boats.
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Just like before,
we can handle this with a question mark.
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Now let's say we want to match toy,
even if it's capitalized.
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Toy boats.
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Regular expressions are case
sensitive by default, so
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we'll need to specify that the T
can be uppercase or lowercase.
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We can tell the interpreter
to accept a capital or
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lowercase T by wrapping both
characters in square brackets.
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Let me show you what I mean.
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I'll put a capital T next
to the lowercase t, and
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I'll surround them both
in square brackets.
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I just added what is called
a character set to the RegEx.
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When the interpreter sees it,
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exactly one of the characters in the set
will match one position in the string.
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We can specify as many characters
as we want between these brackets.
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Let's say we also want to match joy boats,
with a lowercase j.
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I'll add joy boats.
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There's two things to note.
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No matter how many characters
we put in the set,
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the set will match only one
position of the string.
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In this case, it's the first position.
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It also makes no difference how
you order the character set.
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Now I want to give you a challenge.
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I'll type out two more test cases for you.
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See if you can modify the RegEx
to match those as well.
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I'll add a hyphenated version, toy-boats,
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and I'll also add one with a capital B.
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Go ahead and pause this video and
see if you can do it.
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For the hyphenated string,
I'll replace the space in the RegEx
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with a character set containing
both a space and a hyphen.
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Note that the question mark now indicates
that the character set is optional,
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in other words,
the character between the y and
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the b can either be a space,
a hyphen, or nothing at all.
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For the second string,
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I use a character set to match
either a lower or upper case B.
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I've added some additional strings for
you to practice in the teacher's notes.
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Take some time now to give them a try.
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In the next video, I'll show you how to
add a little more power to character sets.
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