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Introduction to regular expressions

From: Foundations of Programming: Fundamentals

Video: Introduction to regular expressions

While strings have some great built-in functionality-- we can convert them to uppercase or lowercase, we can slice pieces out, we can find out if a word exists in the string-- well, sometimes we need a different approach and what we are more interested in is the format of the string. What I mean by that is maybe I want to ask, is it an email address? Not a particular email address, but does it match the pattern for one? Or is it a URL? Perhaps I want to ask if a password has a mixture of uppercase and lowercase letters and special symbols or verify that a credit card number has the correct amount of digits.

Introduction to regular expressions

While strings have some great built-in functionality-- we can convert them to uppercase or lowercase, we can slice pieces out, we can find out if a word exists in the string-- well, sometimes we need a different approach and what we are more interested in is the format of the string. What I mean by that is maybe I want to ask, is it an email address? Not a particular email address, but does it match the pattern for one? Or is it a URL? Perhaps I want to ask if a password has a mixture of uppercase and lowercase letters and special symbols or verify that a credit card number has the correct amount of digits.

Now to do this kind of work many programming languages, including JavaScript, have something called regular expressions built into the language. Regular expressions are strange looking sequences of characters that describe and can match patterns and strings, and nobody finds regular expressions pleasant to begin with. But they're really useful and you should know what they are and what they can do. There is always two parts to working with regular expressions. First, you create the expression that describes the pattern that you're looking for, and next, you try and apply it to something and ask if it matches.

So step one, we are actually going to create the regular expression. We can do it a couple of ways. One is this. It looks a little strange. We are creating a variable. I'm calling it myRE for my regular expression = /hello/. Now this looks a little odd. Where are the quotes? What do we do? Well, this forward slash. Notice it is forward slashes, not backslashes, marking this as a regular expression. This is a JavaScript shorthand for doing this. There is another way of doing it, which would be this.

This is a longer way, but it's doing exactly the same thing, var myRE = and we use the word new RegExp, regular expression, and feed in the string hello into it. What we are actually doing in either of these cases is creating a new regular expression object in JavaScript. JavaScript is what's called an object oriented language, and I'm using the word new in the second example to actually create a new object. Now we haven't explicitly done much with objects yet, but we are going to talk more about them later.

Now the word hello is actually as simple a pattern as you can get. What we are going to do is look for the word hello to exist in another string. So once we've created our regular expression, we then have to test it. So let's say I create a new variable called myString, and this is what I am going to test my regular expression against. So in my if statement, I'll ask if myRE.test, so .test is a built-in method. Again, it's like a function that belongs to this regular expression.

And I'm passing in the variable myString to say test this, check it, does it have the word hello in it. What's going to happen is we get a Boolean response from this. Calling test will return either true or false. In this case, if the word hello exists in myString, which it does, we are going to pop up an alert and say, yes. The regular expression objects have other methods that can return different data. More complex patterns are created by using special characters. So to give you a few examples, if I create a new regular expression variable here, and I'm using the shorthand format which uses the two forwards slashes, one at the start and one at the end, and whatever is between them is the pattern that we are matching on.

The actual forward slashes themselves are not part of the pattern. We've seen how to match on the word hello. Well, if I prefix this by this little character, which is the caret character, we are saying the hello has to be at the start. If instead my pattern was hello$ before the forward slash, it would have to be at the end. Using the plus inside the pattern means that the L would have to appear once or more. So it would match on these different examples. Hello with one l, with two ls, with multiple ls.

hel*o means that it would be a 0 or more. So it'd match all the previous ones, but it would also match heo. The l doesn't have to appear at all. We have the question mark for 0 or 1 match, which will be a bit more restrictive on what it was matching. It would match heo or helo, but would not match more than that. Using the pipe, the vertical bar symbol, means we are actually matching either hello or goodbye. The decimal point is the period, means any character.

Using a /w inside the pattern is matching an alphanumeric or and underscore. \b inside the pattern means we are looking for what's called a word boundary like a space or a new line. You can also use the square brackets to provide a range of characters to match on. In this case, what we are looking for is either crnl or d and then the ope, so it would match on cope and rope and lope and dope. As you're beginning to tell there is a lot of these and there's actually way more than I am showing here, and you can describe extremely complex patterns by stringing them all together.

Here is one for example that describes a US zip code with an optional four digit extension. I'm not going to go through this. Bear in mind, you're going to begin by finding and looking for examples online. If you're looking for regular expressions to match a date or a password or credit card format, there really is no reason that you should be trying to create it yourself from scratch. This next regular expression here is one that tries to match on an email address. In fact, matching an email address is a pretty complex operation and people have been debating for many years on what the perfect regular expression for an email address is.

It looks very complex, but you can actually see there's kind of a repetition of the same kind of thing going on through it. It's really because an email address can follow quite at a lot of complex rules. Email addresses are notoriously difficult to match on regular expressions and this one is certainly not perfect, but it's kind of an example of what you might see and what you might come across. You will find that sometimes with more complex patterns that you're not looking for a perfect match. You're just looking for one that's good enough. And it is worth keeping in mind with regular expressions, particularly if you're brand-new to them, that I wouldn't be at all focused on trying to memorize these formats.

I'd be more focused on understanding the fact that they do exist, what they can be used for, how they're used. The syntax itself is something you don't need to memorize, not now, if probably ever. They are a tool. They're something to be used when you need them, but you'll find that they're built-in to most modern programming languages. They're certainly in JavaScript and they are in almost all of ones you're likely to come across.

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This video is part of

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Foundations of Programming: Fundamentals

61 video lessons · 99316 viewers

Simon Allardice
Author

 
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  1. 4m 15s
    1. Welcome
      1m 17s
    2. Making the most of this course
      2m 8s
    3. Using the exercise files
      50s
  2. 22m 11s
    1. What is programming?
      5m 45s
    2. What is a programming language?
      4m 48s
    3. Writing source code
      5m 34s
    4. Compiled and interpreted languages
      6m 4s
  3. 16m 29s
    1. Why JavaScript?
      4m 45s
    2. Creating your first program in JavaScript
      6m 54s
    3. Requesting input
      4m 50s
  4. 31m 38s
    1. Introduction to variables and data types
      5m 16s
    2. Understanding strong, weak, and duck-typed languages
      3m 51s
    3. Working with numbers
      5m 4s
    4. Using characters and strings
      4m 5s
    5. Working with operators
      4m 47s
    6. Properly using white space
      6m 46s
    7. Adding comments to code for human understanding
      1m 49s
  5. 24m 48s
    1. Building with the if statement
      7m 35s
    2. Working with complex conditions
      4m 9s
    3. Setting comparison operators
      6m 59s
    4. Using the switch statement
      6m 5s
  6. 17m 54s
    1. Breaking your code apart
      4m 1s
    2. Creating and calling functions
      2m 56s
    3. Setting parameters and arguments
      6m 7s
    4. Understanding variable scope
      2m 23s
    5. Splitting code into different files
      2m 27s
  7. 13m 31s
    1. Introduction to iteration
      4m 28s
    2. Writing a while statement
      5m 24s
    3. Creating a for loop
      3m 39s
  8. 19m 28s
    1. Cleaning up with string concatenation
      4m 30s
    2. Finding patterns in strings
      8m 3s
    3. Introduction to regular expressions
      6m 55s
  9. 19m 58s
    1. Working with arrays
      5m 46s
    2. Array behavior
      5m 29s
    3. Iterating through collections
      5m 18s
    4. Collections in other languages
      3m 25s
  10. 10m 50s
    1. Programming style
      5m 55s
    2. Writing pseudocode
      4m 55s
  11. 25m 55s
    1. Input/output and persistence
      3m 6s
    2. Reading and writing from the DOM
      8m 11s
    3. Event driven programming
      7m 47s
    4. Introduction to file I/O
      6m 51s
  12. 24m 25s
    1. Introduction to debugging
      5m 57s
    2. Tracing through a section of code
      7m 5s
    3. Understanding error messages
      3m 21s
    4. Using debuggers
      8m 2s
  13. 14m 16s
    1. Introduction to object-oriented languages
      5m 18s
    2. Using classes and objects
      6m 28s
    3. Reviewing object-oriented languages
      2m 30s
  14. 11m 14s
    1. Memory management across languages
      5m 11s
    2. Introduction to algorithms
      4m 2s
    3. Introduction to multithreading
      2m 1s
  15. 29m 20s
    1. Introduction to languages
      1m 42s
    2. C-based languages
      4m 40s
    3. The Java world
      3m 13s
    4. .NET languages: C# and Visual Basic .NET
      6m 17s
    5. Ruby
      3m 4s
    6. Python
      2m 56s
    7. Objective-C
      4m 3s
    8. Libraries and frameworks
      3m 25s
  16. 1m 2s
    1. Where to go from here
      1m 2s

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