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Welcome to lesson 25 which roughly covers pages 154 to 157 of the automate the boring stuff with Python

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textbook the character that we went over in last lesson lets you match one of many possible groups in

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our regular expression.

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But you may also want to match a certain number of repetitions of the group.

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You may want to match at least one or more appearances of the group or you may want to match the group

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if it appears more than seven times but less than 10 times.

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In this lesson you'll learn the regular expression syntax for matching a specific number of repetitions

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.

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So to start off with I'm just going to import the r e regular expressions module and let's begin with

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the first character.

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The question mark character says match the preceding group 0 or 1 times each.

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It's an optional group.

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It can either appear once or not appear at all.

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Let's use some code.

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I'm going to use a Batman regular expression that I'll save in this bat rechecks variable so I'll call

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compile so that I'll return a regular expression object and the pattern I want this to match will either

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be that man or that woman.

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But in this case I could just make this a bit shorter by just saying adding a W.O. Whoa group and then

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a question mark after it.

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So this question Mark says this group rate here can appear in the in the text zero or 1 times in order

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to match this pattern.

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So if we call this railer expression objects search method and search string like the adventures of

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Batman

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we can see that this regular expression does indeed match this part of the string it found it found

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this pattern in this string and we see it match Batman.

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But this will also match the adventures of that woman

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because the W O group here can appear 1 or 0 times it appears 0 times here and appears once here.

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But if we had something like the adventures of bat.

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Whoa whoa whoa woman that actually doesn't match the pattern at all.

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Because it can only appear as 0 or 1 time.

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So the search method will return none to the variable.

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Using the earlier phone number example we can make a regular expression that looks for phone numbers

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that do or do not have an area code that is we can make the area code optional.

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So to enter this into the interactive shell going to create a regular expression object by calling compile

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and store it in.

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Phone read checks and the pattern for phone numbers.

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We can use that slash D which stands for any numeric digit.

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So we can have 3 digits for the area code three digits for the first part of the phone number and then

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the last four digits of the phone number.

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So this is the phone number rejects that requires a phone number to have an area code.

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So if I had a string my phone number is 4 1 5 5 5 5 1 2 3 4 Call me tomorrow.

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We'll just save that in the variable M-O because it will return a match object.

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We can see that this actually found the phone number with the area code.

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Except this pattern which requires the area code won't be able to match anything.

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If this stream doesn't have the area code so search returns None.

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So the most variable will contain the none value.

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So instead let's create a phone regular expression object that puts this area code into a group and

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then we have the question mark to say this preceding group is optional it can appear once or it can

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appear 0 times.

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And in this case we can see that the search method does return a match object for this phone number

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and also returns a match object for this phone number when it doesn't have an area code in front of

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it.

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And if you ever have to actually literally match a question mark as part of your pattern you can escape

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the question mark with a backslash in front of it.

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The asterisk character is usually called the star character in the context of regular expressions.

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And it means matches zero or more times.

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So let's go back to that Batman regular expression example creating that regular expression.

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And before we had the question mark here which meant that this W.O. could appear one or zero times we

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change that to a star.

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This means this can appear 0 or more times.

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So basically any number of times this will actually match the adventures of Batman string.

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It will also match the adventures of that woman string.

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And since it can accept any number of this w o group because the star means zero or more we say have

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several of that and it would still match that string.

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And if you need to match a literal star that appears as part of the pattern you can escape it with a

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backslash.

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So while the star character means matches 0 or more feet plus character means match one or more.

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So unlike the star which doesn't require its group to appear in the match string the group preceding

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a plus must appear at least once.

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It's not optional so it's created another Batman regular expression object by calling compile

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and let's say that that W.O. group is now required to show up here one or more times.

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This means that matching the adventures of Batman string it won't actually find that this string returns

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or this function call will return none over the adventures of that woman will match it because this

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w o group here of here is once and then also in this example with bat.

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Well we'll Wolman that will also match because that w o group can appear one or more times.

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And if you have to match a literal plus sign that appears in your pattern you can escape it with a backslash

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So let's have a quick example with that escaping.

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Let's say I'm just going to create a rejects object by calling compile and the pattern that I'm looking

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for is a literal plus sign followed by a literal star or asterisk followed by a literal question mark

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that would match a string that looks something like.

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I learned about plus star questionmark reject syntax and those that actually match these characters

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.

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So if you literally want to match these characters that otherwise have a special meaning for regular

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expression strings you can just precede them with a backslash to escape them.

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In fact we could even have something like putting this inside of a group and saying it has to appear

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one or more times by having a plus sign that means 0 one or more times

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and that would match a string that looks like this

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.

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So here it means literally I want you to find a plus sign that appears in the string that I'm looking

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for that's part of the text pattern and this is more of a regular expression instruction saying match

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one or more of this preceding group in the parentheses here.

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So here's a quick recap.

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The question mark means 0 or 1 the star means 0 or more and the plus sign means one or more.

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But let's say if you wanted to match a specific number of repetitions of a group we can do that in Python

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.

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I'm going to create a regular expression object that matches the string.

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Ha exactly three times and we can do that with curly braces with the number three in between them.

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So if we call the search method here we can search for.

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He said Ha Ha Ha

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we can see that this string is matched in the regular expression.

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This is kind of a simple example you might think well couldn't we just have something like.

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Ha ha ha.

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And this would work too.

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This would do the exact same thing as this.

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But the nice part about this syntax is that you could have something like say if you want to match multiple

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phone numbers.

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That either had or did not have the area code in front so that expression would look like this.

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We have the optional area code these are three digits and a dash for which can appear 0 or 1 times.

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And then we have the rest of the seven digits of the phone number.

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And then we get put all of that

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inside of a group and then just say we want this phone number we want to match three phone numbers in

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a row.

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Now that might look a little bit strange.

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Let's say they're also they have a comma that they're separated by.

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Except that comma is also optional.

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You could put that inside of a group and then say that comma is optional.

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So then this phone number red jacks

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would match a string like my numbers are 4 1 5 5 5 1 2 3 4 5 5 5 42 42 and 2 1 2 5 5 5 0 0 0 0.

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A really complicated looking pattern like this could be matched by this regular expression where we're

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looking for three phone numbers in a row which may or may not have the area code and may or may not

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have a comma following it that separates them.

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And remember this pattern would match exactly three of this complicated pattern just like we could have

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passed the string that has three has literally typed out instead of using this curly brace syntax.

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We could have just gotten rid of this and instead copy and paste this entire regular expression three

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times but then at this point it's getting kind of hard to read what exactly we mean from this regular

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expression and just we'd have to spend a lot of time kind of piecing together what this means.

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So the curly brace syntax really helps out on that front.

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Well let's say you wanted to match instead of three.

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You wanted to match a range of possible repetition so anywhere between three and five.

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I can just add a comma and then a second number.

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So then this becomes a minimum and maximum number of repetitions.

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So we can have a string like he said Ha ha ha.

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And that would match.

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We could also have four or five of these hands as well.

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And that would match if we had any more.

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Well this would still match but it would only match the first five.

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Ha ha ha ha's.

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And just like with slices if you leave off this first number or second number.

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In this case this is the same thing as saying zero to five.

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Or if you had three and then a comma followed by no number.

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This is the same as saying three or more.

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So there is an unbounded maximum you can have any number of these as long as it's at least three.

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So let's do a little experiment here.

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I'm going to create a regular expression that matches anywhere between 3 and 5 digits so that slash

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D was the regular expression syntax meaning any numeric digits.

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See I went to match a string that had between three and five of them.

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So I was going to call it and have it search the string.

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One two three four five six seven eight nine zero.

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You can see that what it matched right here was the first five digits.

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I mean it would technically match these five digits but pythons regular expressions start as soon as

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possible as the earliest match they can find is the match that they'll return.

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And also notice it could have matched these first three characters or the first four characters.

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But instead it matched the first five characters.

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And that's because by default regular expressions in Python do greedy matches this means that it tries

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to match the longest possible string that matches this pattern.

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So in an ambiguous situation like this string where it could match any three four or five digits regular

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expressions are going to match the maximum longest possible string.

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Now if you wanted to do a non-greedy match you can specify a question mark after that curly brace.

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So this question mark is different from the question mark that means 0 or 1 when it comes after a pattern

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like this as opposed to directly after a group.

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This means basically do a non-greedy match.

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So let's create this regular expression and then search that same 1 2 3 4 5 6 7 8 9 0 string and you

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see this time it matched the smallest possible string.

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This is a pattern that happens a lot in the regular expression syntax.

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If you add a question mark it'll do a non-greedy match whereas without it it'll just do a normal greedy

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match.

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We're trying to match the longest possible string.

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To recap the question mark matches a group 0 or 1 times the star matches a group 0 or more times.

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The plus matches a group one or more times and the curly braces can match a specific number of times

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.

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And if you put two numbers separated by a comma in between the curly braces you can have a minimum and

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maximum number of times.

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You can also leave out the first or second number inside the curly braces so that you can say that there

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is no met minimum or no maximum greedy matching matches the longest string possible.

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Well non-greedy matching matches the shortest string possible and putting a question mark after the

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curly braces makes it do a non-greedy match by default regular expression syntax.

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Does a greedy match