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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.
In this movie, we're going to look at how to write a regular expression that will match names, and this will help us to illustrate some of the points that I was just talking about in the last movie. So, in order to match a simple list of names, we might think that the easiest solution is just to do any word character one or more times, and that does. That's the most permissive possible thing, and it allows all of our names to match, but we've got a couple problems there. The first is that it also returns a match if we have something like dollar sign Kevin. Now, it didn't match the entire thing, but it did find a match, and this can be a problem for us if we're doing something like form validation.
So we've got the characters in a form field, we submit those to our application, our Web application says, let me run this regular expression, and make sure it's valid. It says, do you find a match? And it says yes, I did find a match. Therefore, it thinks it's valid, and goes ahead and let's the data through. That's not what we really meant. What we really meant is we want only word characters; that symbols like this are not allowed. The way we should handle that is by using anchors at the beginning and end, or some other kind of delimiter, but in this case anchors. Notice now nothing matches; that's because I'm not looking at just a single field. I'm looking at multiple lines. I need to use multi-line anchors, so that those anchors will match at the beginning and ends of the lines.
So now it does correctly match the names, but it no longer matches the example with the dollar sign, but there's another problem here. It matches zero, Kevin as well. That's because, remember, the word character includes digits and underscores. So what we really mean, here, was to have uppercase A to Z, lowercase a to z, inside a character set. Now it makes sure that it only includes alphabetic characters. Now we have some choices to make. Does the first letter have to be capitalized? Right? Should this be a valid one? Right now it is.
If you want to have that first character capitalized, the best way to do that is just, at the beginning here, to say that the character set can be A to capital Z, followed by a mixture of upper and lower case letters. So now we should probably think about some edge cases. I mean, what if we had a name like J.R.? Is that allowed? It so, then we should add up here to our character set that the period is also valid. What about names that might have apostrophes in them? Or hyphenated names? These are the kinds of edge cases that you'll want to consider before you rule out the possibility that there won't be a single case that doesn't match your pattern.
Alright, so that pretty well matches first names. Let's now think about if we had more than one name. Let's say we have George Washington. Right; we no longer have a match there, because there's no space. We could just put a space in here, and say okay, think of this as a match, and it's a perfectly acceptable way to do it. But what if we wanted to actually capture these two? What if we wanted to grab the first name and the last name separately? Well, one way to do that would just to be to take this expression, copy it, and then put a space, and then repeat it, and now we can even capture first one, and capture the second one.
So now we've grabbed each one of those, and we can work with them using backreferences. We saw how to do that earlier. We worked with the Presidents' file, and we actually flipped their names around it, and put Washington in front of George. Okay, but then what about the case when we have a middle name? How should we handle that? John Quincy Adams is a President as well, but it no longer matches our pattern, because we've only allowed for one space to be in here, and this has two spaces. Well, it kind of depends on what your purposes are. If your purpose is just simply to grab Johnny Quincy, and throw it all in one field together, and you're okay with that, then you could put a space here, and just say, alright; if you encounter a first space, it belongs to the first name. There is no space allowed in the last name, so the only other space that's allowed is the one that's in between, delimiting the two. Therefore, John and Quincy become part of the first capture, and Adams becomes part of the second capture.
What if you wanted to capture it separately, though? What if you wanted to actually grab the middle name if it exists? Well, then we don't want to allow this there. Instead, what we want to do is we want to say that we have the same expression in the middle, and that will then match John Quincy Adams, but it doesn't match George Washington anymore. In the process, we have broken that one. Now, you might think, well let's just put an optional here. That will make that middle name optional, but it still didn't match it. Do you see why? It's because the space here -- if this is optional, we're saying that there should be two spaces, right? Space, space. And there is not space, space in between George Washington.
If we did add another space, look at that. Now it suddenly matches. So the problem is, so not only do we have our capture group for the middle name, but then we actually need to make what's optional the space as well. See how that works? It's a little awkward to read with the highlighting, but here's my capture group. It's just the middle name, and then what's optional is that same capture group, but with a space included. That's what's optional, and of course, it's a good idea to make that one non-capturing, so that what's really being captured is what we intend, and the optional grouping -- the one that includes that extra space -- is not being captured.
Now, you can take this further, and you can make more choices. International names probably don't fit these patterns at all. There is a possibility that someone could have comma, Junior, or comma, Senior after their last name. You might want to allow for those kinds of cases, but you get the idea. What I want you to see is that there's no single solution to a problem. It's always a set of judgment calls, and always a set of thinking about the data that you're actually trying to match, and what those edge cases are. And if you think about those things, you'll be able to use the basic regex rules that we've written to come up with something that will work for you.
I want to show you one last example before we leave names behind. What if we had the President Martin Van Buren? What would our regular expression do? It would work; it still does match, but it would capture Martin as being the first capture, Van as being the second capture, and Buren as being the last capture. That seems to be correct in terms of the regular expression that we've written. However, Van Buren is actually his last name. This is a fundamental problem in regular expressions. They're not going to be able solve all of your problems for you.
John Quincy Adams, Quincy is the middle name; Martin Van Buren, Van is part of the last name, but there's no way for it to tell the difference between those two. Now, you could try to write a regular expression where you made Van into a special exception, and said, if it's Van, then it get stuck in the last name. And maybe you could even go through and come up with a list of all of those possibilities of what could potentially be in that last name, but I think that's tough to do. More likely, if you're trying to do some kind of data processing, this just requires a human to go back at the end, and review what your regular expression did, and make sure that special cases like this didn't fall through the cracks.
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