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to look for the username at the
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beginning of the string from the user
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and we're looking for the edu literally
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at the end of the string from the user
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those dots are a little too encompassing
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right now I am allowed to type in more
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than the single at sign why because at
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is a character and Dot means any
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character so honestly I can have as many
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at signs is this thing at the moment as
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I want for instance if I run python of
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validate dopy maen harvard.edu still
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works as expected but if I also run
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python of validate dopy and incorrectly
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do mail harvard.edu that should be
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invalid but it's considered valid
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instead so I think we need to be a
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little more restrictive when it comes to
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that dot and we can't just say oh any
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old character there is fine we need to
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be more specific well it turns out that
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regular Expressions also support this
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syntax you can use square brackets
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inside of your pattern and inside of
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those square brackets include one or
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more characters that you want to look
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for
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specifically alternatively you can
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inside of those square brackets put a
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carrot symbol which unfortunately in
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this context means something completely
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different from match the start of the
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string but would be the complement
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operator inside of these square brackets
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which means you cannot match any of
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these characters so things are about to
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look even more cryptic now but that's
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why we're focusing on regular
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Expressions on their own here if I don't
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want to allow any character which is
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what a DOT is let me go ahead and I
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could just say well I only want to
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support a or B's or C's or D's or E's or
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FS or G's I could type in the whole
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alphabet here plus some numbers to
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actually include all of the letters that
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I do want to allow but honestly a little
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simpler would be this I could use a
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carrot symbol and then in at sign which
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has the effect of saying this is the set
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of characters that has everything except
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an at sign and I can do the same thing
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over here instead of a DOT to the right
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of the at sign I can do Open
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Bracket carrot at sign and I admit
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things are starting to escalate quickly
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here but let's start from the left and
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go to the right this carrot outside of
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the square brackets at the very start of
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my string as before means match from the
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start of the string and let's Jump Ahead
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the dollar sign all the way at the end
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of the regular expression means match at
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the end of the string so if we can
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mentally tick those off as
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straightforward let's now focus on
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everything else in the middle well to
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the left here we have new syntax a
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square bracket another carrot an at sign
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and a closed square bracket and then a
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plus the plus means the same thing as
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always it means one or more of the
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things to the left what is the thing to
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the left well this is the new syntax
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inside of square brackets here I have a
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carrot symbol and then an at sign that
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just means any character accept an at
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sign it's a weird syntax but this is how
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we can express that simple idea any
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character on the keyboard except for an
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at sign and heck even other characters
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that aren't physically on your keyboard
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but that nonetheless exist
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then we have a literal at sign then we
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have another one of these same things
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square bracket carrot at close bracket
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which means any character accept an at
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sign then one or more of those things
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followed by literally a period
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U so now let me go ahead and do this
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again let me rerun python of validate
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dopy and test my own email address to
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make sure I've not made things worse and
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we're good now let me go ahead and clear
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my screen and run python of validate
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dopy again and do
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mailin at harvard.edu crossing my
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fingers this time and finally this now
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is invalid why I'm allowing myself to
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have one at sign in the middle of the
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