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Learn how to find and manipulate text quickly and easily using regular expressions. Author Kevin Skoglund covers the basic syntax of regular expressions, shows how to create flexible matching patterns, and demonstrates how the regular expression engine parses text to find matches. The course also covers referring back to previous matches with backreferences and creating complex matching patterns with lookaround assertions, and explores the most common applications of regular expressions.
So, we have completed the first two tasks in formatting A Midsummer Night's Dream by using regular expressions. Now we are going to try third, and harder, task. The director of the play comes to us and says that really, ideally, she would like each of these actor's lines to be changed, so that instead of being the character's name, and then a new line, that it would have a colon, and a space. And that each of the lines after that would be indented, 1, 2, 3, 4; four spaces, just like that. Now, to go through all 3000 lines of the play by hand would take a long time to do that, but with regular expressions we don't have to.
So we've got a number of different kinds of lines that we are going to have to deal with. So let's do these one at a time. First, let's take care of the matching the lines that are the character name, and put the Colon after it, and remove the line return that's there. We have seen something similar this before. We just have to say that we want to match all of the capitals, A to Z, and more than one of them. Of course, that will find it, but we want to find it not when it's inside the stage directions; we want to find it only when it's at the beginning of the line. So we can use our anchors for that; when the entire line consists of A to Z.
So there we are. We found -- let's jump back here -- Theseus, Hippolyta, Theseus, and so on. So let's try it, and it finds the first one, second one, and so on. So it is working. So that works, and gets just that line that we want. And we know how to replace it; we can just use our parentheses around it to capture it. We can use our backreference here to make sure that we find it, and then we are going to put a colon, and a space after it. And we also need to get rid of this line return, right? That line return has got to disappear. How do we get rid of it? Well, the way that we get rid of it is that we match for it, but then we don't put it in our replacement string down here.
We will leave it out, so it gets matched up here, but then it's not replaced when we replace it. Let me show you what that does. Let's jump up here at the top. We will find it, so here we are. Let's jump over here. Let's find the next one. We'll want to take out that trailing anchor now, and then we will say Return; here we are. It's finding the entire name, and the line return too. Now we hit Replace & Find. You can see that it did it right there. So now we have just a colon, and the space after it.
So that's the technique is, if we want to remove a line return, we find it, and make it part of the match, but not part of the capture, and not in the replacement string. So let's just try the next one. Find & Replace, Find & Replace, and they're all working correctly. We can finally just say Replace All, knowing that we have got them all. It will take a second, and you can see it replaced 492 of them. So it would take a long time for us to do that by hand, but regular expressions made it really, really fast. So now let's try and accomplish the second portion, which is that we are going to try and put four spaces in front of each of these lines that is not a character name.
We know how to find a character name, right? We have already written that regular expression that will find the character name. Now what we want to do is find something that is not that character name, and incidentally, we can change it now, since it's been changed to a colon, space in front of it. So how do we find something that is not that; that does not match that pattern? To do that, we need to use negative lookahead assertions. Remember, that's the powerful point of negative lookahead assertions, is that they allow us to specify that something is not a regular expression. So to do that, we put capture around the whole thing, and then we use question mark, exclamation point to say that it is not equal to this.
So that's what I am asserting is that it's not equal to that line. Let's jump here, and let me just use the Command+G. It's a little easier to see when I do it this way, instead of from the other window, because we can see where the cursor moves to. Do you see it skip across? It skipped pass the name of the character, down to the line below it, and it matches all of those lines. And then it skips across that name as well, and so on. Now that takes care of all of these blocks just fine, but what about the case in which I have stage directions? I don't want to put spaces in front of there, so I need to change my regular expression, and I am going to put parentheses around this portion, and put in an alternation.
So it's either -- if it starts with first name, it's not that, or also not if the character at the beginning is a square bracket. And of course, if we're putting a square bracket inside a character set, we need to use the backslash there, and the reason I am using a character set is because, if we jump up here, there's also another character we know we want to omit, which is the dash. Let's put that in as well. So if it's either a square bracket, or a dash, then we also want to ignore it. So there we go. I am skipping that one, it skips that one; it is matching, however, these new lines.
Those line returns in there. We don't want it to match those, so let's put in another alternation that says that it is not going to be a new line, and I noticed that it also matched up here for Act, Scene, and Location, which is going to occur a few times. So let's go ahead and say that it can't be those, and we can just put in the literal text. It can't start with Act, the line can't start with Scene, and the line can't start with Location. Now, it's possible that those words could be used inside the text, but I think it's unlikely. To make sure, I am going to go ahead and put a colon after them, and that will make sure that it really is in this context.
And you could even put the space that should be after it as well. Alright, so now let's try it. Let's jump back up here. We will use Command+G, and it jumps straight down here to the word Draws. See that? So it's now finding the right words. I am using Find to make sure that I have got the right stuff, and you can just go through these so you feel confident about it. Now, let's go back to our Find. Once we feel confident, now we need to put in our replacement string. We know the replacement string is going to be four spaces, but how do we insert it there? If you remember when we talked about lookahead assertions, lookahead assertions are zero-width.
So since we only have a lookahead assertion here -- that's all we have -- it is zero-width. Therefore, the cursor -- the position of the cursor -- will be right after the beginning of the line; right after the anchor tag. So we do our replacement here, space, space, space, space; four spaces. What it's going to do is it's going to take that zero-width match, and replace it with four spaces, essentially doing an insert. We saw this back in the chapter on lookahead assertions. So let's try it. Let's jump back to the top here, and I am going to come right below this line, because A Midsummer Night's Dream, and by William Shakespeare also would match, but I am going to start below them. And let's do Next, and then let's do replace. There it worked.
Replace & Find, Replace & Find; see how that works? And like I said, when you feel confident about it, and you feel like it's doing what you want, then you can just hit Replace All. Now, Replace All is going to wrap around, and of course, it is going to match these. I could have put them in as an exception, or since I know they are at the top of the document, I can also just take them out by hand. So now we can scroll down, and we can see that we have very much changed the formatting of the document. And we did it for every single line, all of the way through, very consistently, and we didn't have to enter them all by hand, all by using the power of regular expressions.
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