[MUSIC] All right, in this video we'll look at just the basics of something called regular expressions. Regular expressions themselves are a really big field of study, way too much to fit into a single video, or even into a single lesson perhaps. So we're just gonna very much touch on the basics that you will need to accomplish your project. So, by the end of this video you'll be able to write regular expressions to match String patterns in Java, as well as write regular expressions to split strings. And this is the functionality you'll need for your project. So again, going back to the problem here that we left off in the last video with, we wanted to be able to split the sentence apart using not just a single space but any number of spaces to use as a split delimiter. And we said that the key to this was going to be regular expressions. So let's look at what regular expressions are and how we can use them in Java. And we're gonna build them sort of from the ground up. In a nutshell, a regular expression is just an expression of a pattern that we are trying to match, and from the most basic point of view, a regular expression can be just a simple character. And this is why it worked when we passed a space character into our split method, because we are telling Java, match literally a single space as the pattern you are trying to look for. So Java matched a single space, and every time it saw a single space, it said, okay, here's where I want to split. But that's pretty, pretty basic, there's not too much we can do with that. So we can also combine regular expressions to form more complicated ones and there are three ways that we can combine them. We can combine them with repetition. We can combine them with concatenation, in other words, putting them together. We can combine them with alternation, meaning either/or, choose one expression or the other. We're going to start by looking at repetition because that's what we're after. We're trying to figure out how can we match not just one space but any number of spaces. So we saw that we can match one space just by passing it in as a string. If we wanna match more than one string, we can use the plus operator in a regular expression. So this is the first way we can do repetition in our regular expression. If we put a plus after some regular expression, this case just a space, what we're indicating is that we want to match one or more of these regular expressions in a row, so in this case we're going to match against one or more spaces in a row. So now when Java does the split, it always tries to match as many spaces as possible when it does the match. So it will match just one space but in this case it will also match two. And it will consume those two spaces as the token it matched and give us back just the text on either side of those two spaces. Now it can get a little bit tricky to think about regular expressions when we're splitting strings. We find it's much easier to think about regular expressions in terms of matching, what it actually gets matched. So, we're going to transition over into the way you'll be mostly using regular expressions in the project. So, let me introduce to you a little bit of the code from the project, and it's this abstract class called Document, which you're going to use to represent a document of text. It has a member variable called text that stores all the text in a particular document. Now notice that this class is abstract, so we'll be providing you with some of the code that makes the Document class but we'll also have some abstract methods in there that you're going to have to override when you write your code. You'll be writing subclasses of this Document class. One of the methods that we are providing for you is this method called getTokens. And what the getTokens method does is it takes a pattern which is a regular expression of the tokens that you are looking for. So this is a regular expression that describes what kind of tokens you want to get back from this method. And then the method will return a list of those tokens that match that regular expression. So, let's apply this back to our one or more spaces example. If we have a document that contains the text Hello, space space, hello? And we call the getTokens method on it, what it's going to do is it's going to find those two spaces between the words. And it will just return us a list containing a single string with those two spaces. So again, now we are preserving the text that matches the regular expression, rather than getting rid of it like we did in the split method. So we're gonna use this getTokens method to illustrate some of the other ways we can combine regular expressions. The second way we're gonna look at is called concatenation, and this is the idea of just putting regular expressions together one after another. So if I have a slightly more complicated string, say I have a document with the string, splitting a string, it's easy as 1 2 33! Right? And I now call getTokens using the regular expression IT, this is actually concatenation of two regular expressions. It's a concatenation of the regular expression I and T. And this is going to match whenever it sees those two regular expressions, the I, T, one after another. So in this case it will match on I, T, in splitting, and it will match on I, T in it's, we get back a list of just the two strings, "it", "it". We can combine concatenation and repetition, so now I've got a regular expression that has i followed immediately by one or more ts. So let's see how this changes what we're going to get back from the get tokens. This time, we not only get the i t out of splitting, but because we've asked for one or more ts, Java is going to be greedy and consume as much of that text as it can that matches that regular expression. So here it will match i t t, and then that i after i t t doesn't match anymore so it will stop and just say ok, i t t, that's the first match that I found. The next match it finds again is just that I-T in it's, once again. If you're not sure of the grouping of the precedence order, you can always use parentheses to do explicit grouping in regular expressions. So if I wasn't sure if that plus was going to apply to just the T or to the I-T, I could have put the t in parenthesis. It turns out that plus binds tighter than concatenations, so it applied only to the t. But when in doubt, just go ahead and use those parenthesis. There's another form of a repetition operator, a star. It's very similar to the plus, but this time it means zero or more. So this regular expression, if you might want to take a moment to think about, what this regular expression is going to match against in our string that we have there. Alright, so it's going to match an i followed by zero or more ts and in this case it's going to return that list that you see there and I've highlighted the places in that document string where it matches that regular expression. So you can see that because it matches zero or more Ts sometimes it matches just a plain I. So we've talked about concatenation, putting two regular expressions together. Repetition, repeating a regular expression. And now we have this notion of alternation or an either or kind of choice. The way we represent or in a regular expression is just with a vertical bar. So what you see here is a regular expression that actually combines concatenation and alternation and will match either the pattern I-T or the pattern S-T. So if we apply that regular expression against this string, you can see the text highlighted in blue is what's going to be returned, which is just it, st, and it. Another way we can do alternation is by creating these things called character classes. So when we put characters in square brackets inside the regular expression that we're looking for, what Java will match is any character inside that set. So here I have the character class 1, 2, 3. Java is going to match any single character 1, 2, or 3, and I'll get back the list that you see there of 1,2,3,3. Notice that it matched the 3s separately because that character class says that I want a single character that's either a 1 a 2 or a 3, not more than one character, just a single character at a time. You can think about how you might change that to be multiple characters. And that's just a foreshadowing of what's coming up in our concept challenge. We can also express inside these character classes ranges using a dash. So if I say one dash three inside a character class that means match any number, any numeral, any character between one and three. So in this case it matches one, two, three, and three again. I can use this same range functionality with letters. So I can say match any character between a little a and a little f. And that will match all the characters in this string that are alphabetically between little a and little f. And I'll get back this. And then finally, the last piece of these regular expressions that I want to introduce to you is this carat. And what this carat does inside a character class is it excludes all of the characters inside that character class. So you're basically saying, match anything that is not one of these characters. So let's break down what this regular expression that I've written here actually does. I'm saying that carat, which means NOT, so none of these characters should match. So I don't want any characters that are between little a, and little z. I don't want a 1, I don't want a 2, I don't want a 3, and I don't want a space. So all of those characters, don't match them. Anything else, please match them and give me what you find. So when I use this regular expression to get Tokens what I'm going to get back is all the single characters that are not one of the character class that I just set out. And so that includes the capital S, remember in Java lower case letters and upper case letters are represented differently. So the capital S, the comma, the apostrophe, the exclamation point, and the capital R all match the regular expression. So those are the very basics of regular expressions. We've got a concept challenge, and some support videos, but this should be enough to get you going on the project.
