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Beginning and Ending of Strings

2:01 with Joel Kraft

It's important to be able to specify that a regular expression starts matching at the beginning of a string and match all the way to the end. Learn more in this video.

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Practice

Copy both the Match and the Exclude set of test strings from each exercise below 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 Match set and exclude the ones in the Exclude set.

1 )

Match:

tart

Exclude:

start
tartan

2 )

Match:

img_01.jpg
img_02.png
img_03.gif
img_04.png
img_05.gif
img_06.jpg

Exclude:

6_img_07.gif
mov_01.avi

3 )

Match:

projector
protractor
projected

Exclude:

proctor
my projector
projecting
projectorlight

4 )

Match:

3456
34567

Exclude:

23456
345678

Notice this string I typed in the last video, wwwgoogle.com. 0:00

Even though we've grouped the www dot together, 0:06

the regex is still finding a partial match in this test string. 0:09

As you can see, it does this by starting to match the pattern, 0:14

later in the string, at the fourth character. 0:18

We can see the opposite problem by typing another test string, 0:20

with extra characters at the end. 0:24

If we're trying to match Google.com, neither of these strings are valid. 0:28

We can tell the parser to only consider these a match if the entire string 0:33

fits our pattern, and to reject partial matches like these. 0:37

You will see these characters often in regex's. 0:42

They are very useful in ensuring, 0:45

you are only matching at the beginning or ending of a string. 0:47

While other regex characters have represented characters and 0:51

strings, these only represent location. 0:54

As I mentioned in the first video, 0:59

make sure you have this checked, match at line breaks. 1:01

This lets us treat each line as a separate test string rather than one block. 1:05

To tell the parser to only start matching at the beginning of test strings 1:10

I'll put a caret at the beginning of the regex. 1:15

You see, that excluded wwwgoogle.com because 1:19

if www is present, it must be followed by a dot. 1:24

Now to specify the string must end with net or 1:29

com, with only one m, I'll put a dollar sign at the end of the expression. 1:32

Now all of the valid strings are matched. 1:38

You'll probably use a carat and dollar sign often in your regular expressions to 1:41

eliminate strings that contain the pattern you're matching, but 1:46

as a whole would be considered invalid. 1:49

Find some additional practice in the teacher's notes below. 1:53

Next, let's start using everything we've learned in a real project. 1:56

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