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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.

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