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Introduction to regular expressions

From: JavaScript Essential Training

Video: Introduction to regular expressions

JavaScript, like many programming languages, if not all of them these days, has regular expressions built into the language. Now, regular expressions are odd-looking sequences of characters that describe and can match patterns and strings. They let you verify that a credit card number has the right amount of digits, or that an email address, or URL matches a basic pattern, or that a password has a mixture of uppercase and lowercase letters, or just define certain words in a larger string. Now nobody finds regular expressions pleasant to begin with, but they are very useful, and there is always two parts to them: first you create the regular expression that describes the pattern that you are looking for, and then you apply it to something else to see if it matches.

Introduction to regular expressions

JavaScript, like many programming languages, if not all of them these days, has regular expressions built into the language. Now, regular expressions are odd-looking sequences of characters that describe and can match patterns and strings. They let you verify that a credit card number has the right amount of digits, or that an email address, or URL matches a basic pattern, or that a password has a mixture of uppercase and lowercase letters, or just define certain words in a larger string. Now nobody finds regular expressions pleasant to begin with, but they are very useful, and there is always two parts to them: first you create the regular expression that describes the pattern that you are looking for, and then you apply it to something else to see if it matches.

So step one, we describe the regular expression. We can create a variable in JavaScript. I've created one called myRE for my regular expression and set it equal to /hello/. This is almost like creating a string variable except we don't use quotes but forward slashes to mark the beginning and the end. Now, this is actually a shortcut in JavaScript for the longer version, which we could use the new keyword, new RegExp, or regular expression object and pass in hello.

These both would create the same variable. It's the same way that we can create a new array object or use square brackets as a shortcut, we can make a new regular expression object or use the forward slashes as a shortcut. Now this is about as simple a pattern as you can get. It's just going to look for the word hello to exist somewhere in a string to be matched against. So I can then create a new string, in this case call it myString, and what I am going to do is call the test method of my regular expression against my string.

So if myRE.test, pass myString in, does that word hello occur? Yes, it does. We'll pop up an alert. Now this is case sensitive. Calling test will just return true or false. If you called search instead, you would actually return the position of the first match. Now, a complex pattern than just single words are created by using special characters, so as an example if I created a regular expression variable with the carrot symbol, this would denote the start of the word or the string that we are matching it again.

So hello would have to appear right at the start of the string. On the flip side, hello with a dollar sign at the end means hello would have to appear at the end of the string. And we can get even more specific. If I use a plus sign somewhere in the regular expression, that means the previous character, in this case L, has to appear once or more, which in this case would match for hello with one L, hello with two Ls or hello with a dozen or 500 Ls.

If I instead used an asterisk, that would be zero or more times, so the previous character L would have to be there zero or more, which means it would also match on h-e-o, so no L at all. And then we can also use a question mark, which means just zero or one, so h-e-o would match, h-e-l-o would match, but any more Ls than that would not match. If I use the pipe, it means either/or. In this case, it'd be true if the string contained either hello or goodbye.

If I use the point here, it means any character, which would much a whole bunch of different things. Now you will also see the backslash used a lot with regular expressions. So \w used as one little piece here means that this must be an alphanumeric character or an underscore. \b means a word boundary, like a space or a new line, which means here that hello would have to appear after a space or after a new line as a word by itself and not as part of another word.

And very often you'll also see the square brackets being used to denote a range of characters to match on. So in this case, I've got c-r-n-l-d inside square brackets. That means any of those letters followed by o-p-e will be regarded as true, but if I have a different letter, it will not. Now as you're probably beginning to tell, there's a lot of these things, and there is way more than I can show you here. What we start to do is describe more complex patterns by starting to string them together.

So as an example, here is one that would check for a valid format for a US ZIP code with an optional four-digit extension. We are surrounding it with the forward slashes at the start and end. Then we are using the carrot at the start to denote the start of the string and saying what characters allowed, how many characters there can be, and then the question marks to denote whether things are optional or not. Now, bear in mind you're likely to begin by finding examples of these online. If you're looking for a regular expression to match a credit card number format or a date or a password, there is really no reason you should be writing it yourself from scratch.

Now, oftentimes the best regular expression is a matter of some considerable debate. People have been arguing for many years on what the so-called ideal regular expression for an email address would be. Now, here is actually very simple one. What this is saying is that the first part of the line begins with a carrot and then after than we can have multiple letters, digits, periods, dashes and underscores. Then the @ must exist and then more of these matches where we can't use the special characters, then at least one, and then it must end with the top-level domain, which can be two to four letters like.com.net.org or a country code like UK.

But here's the thing, email regular expressions are notoriously difficult, and this one actually isn't correct. It won't allow long top-level domains like.museum, which might be rare, but certainly does exist. And it doesn't allow plus signs in the email, which can be used by some mail systems. In fact, the range of permissible email addresses is more complex than you might think, and if you include non-Western character sets, you should get into more complex regular expressions that are dozens, or even hundreds, of lines long. There really is no perfect regular expression for validating email.

There is just a variety of ones that are good enough, and this is worth bearing in mind with regular expressions. There is a lot of knowledge out there. Right now, what you need to understand is what they are and how they used. Regular expression syntax is not something you need to memorize just yet, if you've never come across them before. They are a tool, and there are something to be used when you need them.

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This video is part of

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JavaScript Essential Training

56 video lessons · 101148 viewers

Simon Allardice
Author

 
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  1. 3m 28s
    1. Welcome
      1m 1s
    2. What you should know
      1m 44s
    3. Using the exercise files
      43s
  2. 15m 41s
    1. Introduction to JavaScript
      8m 6s
    2. Creating your first JavaScript
      2m 13s
    3. Getting to know the tools and applications
      5m 22s
  3. 56m 8s
    1. Understanding the structure of JavaScript code
      7m 9s
    2. Where to write your JavaScript
      3m 56s
    3. Creating variables
      6m 21s
    4. Working with conditional code
      5m 44s
    5. Working with operators
      13m 28s
    6. Sending messages to the console
      2m 59s
    7. Working with loops
      8m 1s
    8. Creating functions
      8m 30s
  4. 36m 13s
    1. Working with arrays
      7m 57s
    2. Working with numbers
      6m 13s
    3. Working with strings
      8m 27s
    4. Working with dates
      5m 38s
    5. Working with objects
      7m 58s
  5. 9m 6s
    1. What is the DOM?
      5m 49s
    2. Working with nodes and elements
      3m 17s
  6. 25m 17s
    1. Accessing DOM elements
      11m 3s
    2. Changing DOM elements
      5m 42s
    3. Creating DOM elements
      8m 32s
  7. 24m 45s
    1. Introduction to JavaScript event handling
      8m 16s
    2. Working with onClick and onLoad events
      7m 36s
    3. Working with onBlur and onFocus events
      2m 36s
    4. Working with timers
      6m 17s
  8. 21m 41s
    1. Common JavaScript errors
      7m 14s
    2. Using Firebug
      4m 7s
    3. Going through a debugging session
      10m 20s
  9. 10m 13s
    1. Accessing form elements
      4m 20s
    2. Preventing a form from being submitted
      2m 36s
    3. Hiding and showing form sections
      3m 17s
  10. 9m 49s
    1. CSS and JavaScript
      3m 46s
    2. Removing and applying CSS classes
      2m 16s
    3. Changing inline styles
      3m 47s
  11. 19m 44s
    1. Understanding JavaScript style
      7m 39s
    2. Minifying your code
      4m 28s
    3. Using JavaScript code checkers
      7m 37s
  12. 22m 24s
    1. Introduction to JavaScript libraries
      3m 17s
    2. Linking to multiple JavaScript files
      2m 11s
    3. Introduction to jQuery
      12m 7s
    4. Using a content distribution network to deliver JavaScript files
      4m 49s
  13. 17m 35s
    1. JavaScript in HTML5
      9m 37s
    2. Using Modernizr
      3m 2s
    3. Using Strict Mode
      4m 56s
  14. 33m 3s
    1. Knowing the JavaScript to avoid
      6m 35s
    2. Introduction to regular expressions
      6m 56s
    3. Working with AJAX
      10m 44s
    4. Working with objects and prototypes
      8m 48s
  15. 21m 10s
    1. Example: Countdown
      8m 3s
    2. Example: Resize
      5m 47s
    3. Example: Accordion
      7m 20s
  16. 4m 58s
    1. Where to go from here
      4m 0s
    2. Goodbye
      58s

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