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Introduction to Regular Expressions

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Beginning and Ending of Strings

2:10 with Alena Holligan

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.

Practice

Copy both the Match and the Exclude set of test strings from each exercise below into 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

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    Notice the 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 later in the string, 0:14
    at the fourth character. 0:19
    We can see the opposite problem by typing another test string with extra 0:21
    characters at the end. 0:26
    Google.com with a couple extra m's. 0:30
    If we're trying to match google.com, neither of these strings are valid. 0:34
    We can tell the parser to only consider these a match if the entire string 0:39
    fits our pattern and to reject partial matches like these. 0:44
    You'll see these characters often in regular expressions. 0:50
    They are very useful in ensuring that you're only matching at the beginning or 0:54
    ending of a string. 0:59
    While other reg ex characters have represented characters in strings, 1:00
    these only represent location. 1:06
    As I mentioned in the first video, make sure you have the m flags set. 1:09
    This lets us treat each line as a separate string, rather than one block. 1:13
    To tell the parser to only start matching at the beginning of a test string, 1:19
    I'll put a caret at the beginning of the regex. 1:23
    You see that excluded wwwgoogle.com, 1:29
    because if www is present, it must be followed by a dot. 1:32
    Now, to specify that the string must end with .net or 1:38
    .com with only one m, I put a dollar sign at the end of the expression. 1:43
    Now all the valid strings are matched. 1:49
    You'll probably use the caret and dollar sign often in your regular expressions 1:53
    to eliminate strings that contain the pattern you're matching. 1:58
    But as a whole would be considered invalid. 2:02
    For some additional practice, 2:05
    once again check the notes associated with this video. 2:07

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