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What are regular expressions?

From: Using Regular Expressions

Video: What are regular expressions?

Let's begin by answering the question, what are regular expressions? The name is opaque; it doesn't really give its meaning away. Regular expressions are all about text. Take just a moment and realize how much text is all around you in our modern digital world: email, news stories, text messages, stock market data, computer code, contacts in your address book, tags of people in photographs--all these things are text. Regular expressions are a tool that allows us to work with these text by describing text patterns.

What are regular expressions?

Let's begin by answering the question, what are regular expressions? The name is opaque; it doesn't really give its meaning away. Regular expressions are all about text. Take just a moment and realize how much text is all around you in our modern digital world: email, news stories, text messages, stock market data, computer code, contacts in your address book, tags of people in photographs--all these things are text. Regular expressions are a tool that allows us to work with these text by describing text patterns.

So a regular expression is a set of symbols that describes a text pattern. Now it's the singular. When we see it pluralized, what we're talking about is the formal language of these symbols that needs to be interpreted by a regular expression processor. We'll talk a bit more about processors in just a moment, but the processor is what's going to use those symbols to allow us to match, search, and manipulate text. Now let's also take a moment and talk about what regular expressions are not. They're not a programming language. They may seem similar because they are a formal language with a defined set of rules that gets a computer to do what we want it to do.

Most programming languages use regular expressions and programmers probably use them the most, but there are no variables and you can't add 1+1. It's not a programming language. What they are are symbols that describe a text pattern, and that's it. Frequently, you'll hear them regex for short. Sometimes you'll see it written with a p at the end, but that's really not that common; more often you see it without. You'll hear me say regex throughout this tutorial, and you'll even hear it pluralized as regexes. It's just a lot shorter and simpler to say than regular expressions, which is a bit of a mouthful.

Next let's talk about ways that you might use regular expressions to work with text. You might use them to test if a phone number has the correct number of digits, if an email address is in a valid format. You could search a document for color spelt either with or without the U. You could search a document and replace all occurrences of Bob, Bobby, or "B." with Robert, count the number of times in a document that training is immediately preceded by the words "computer," "video," or "online," only in those cases, only training when those words precede it.

You could use it to convert a tab-delimited file into a comma-delimited file or to find duplicate words in a text. In each of these cases, we're going to use a regular expression to write up a description of what we're looking for using symbols. In the case of a phone number, that pattern might be three digits followed by a dash, followed by three digits and another dash, followed by four digits. Once we've defined our pattern then the regex processor will use our description to return matching results, or in the case of the test, to return true or false for whether or not it matched.

Now that word matches is a keyword. We're going to be using it a lot. A regular expression matches text if it correctly describes the text. You can also flip it around and say that text matches a regular expression if it is correctly described by the expression. So you hear it both ways. So whether if something matches your regex, that's the verb we're going to be using a lot. Does it match, does it not match? We're going to learn to write all these examples and more, but before we begin learning the symbols that are required to write these expressions, let's first take a look at the history of regular expressions and get set up with an environment where we can test them out.

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This video is part of

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Using Regular Expressions

59 video lessons · 11676 viewers

Kevin Skoglund
Author

 
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  1. 2m 18s
    1. Welcome
      56s
    2. Using the exercise files
      1m 22s
  2. 19m 55s
    1. What are regular expressions?
      3m 20s
    2. The history of regular expressions
      6m 40s
    3. Regular expression engines
      2m 44s
    4. Installing an engine
      4m 5s
    5. Notation conventions and modes
      3m 6s
  3. 21m 23s
    1. Literal characters
      6m 39s
    2. Metacharacters
      2m 1s
    3. The wildcard metacharacter
      4m 31s
    4. Escaping metacharacters
      4m 53s
    5. Other special characters
      3m 19s
  4. 31m 26s
    1. Defining a character set
      5m 49s
    2. Character ranges
      4m 49s
    3. Negative character sets
      4m 53s
    4. Metacharacters inside character sets
      5m 12s
    5. Shorthand character sets
      6m 30s
    6. POSIX bracket expressions
      4m 13s
  5. 36m 38s
    1. Repetition metacharacters
      7m 17s
    2. Quantified repetition
      6m 59s
    3. Greedy expressions
      6m 27s
    4. Lazy expressions
      6m 46s
    5. Using repetition efficiently
      9m 9s
  6. 20m 24s
    1. Grouping metacharacters
      4m 14s
    2. Alternation metacharacter
      4m 54s
    3. Writing logical and efficient alternations
      7m 33s
    4. Repeating and nesting alternations
      3m 43s
  7. 19m 19s
    1. Start and end anchors
      7m 21s
    2. Line breaks and Multiline mode
      4m 41s
    3. Word boundaries
      7m 17s
  8. 23m 33s
    1. Backreferences
      8m 57s
    2. Backreferences to optional expressions
      3m 51s
    3. Finding and replacing using backreferences
      7m 16s
    4. Non-capturing group expressions
      3m 29s
  9. 32m 31s
    1. Positive lookahead assertions
      6m 39s
    2. Double-testing with lookahead assertions
      7m 16s
    3. Negative lookahead assertions
      6m 10s
    4. Lookbehind assertions
      6m 26s
    5. The power of positions
      6m 0s
  10. 13m 13s
    1. About Unicode
      4m 19s
    2. Unicode in regular expressions
      4m 41s
    3. Unicode wildcards and properties
      4m 13s
  11. 1h 55m
    1. How to use this chapter
      5m 38s
    2. Matching names
      6m 33s
    3. Matching postal codes
      8m 54s
    4. Matching email addresses
      5m 0s
    5. Matching URLs
      8m 1s
    6. Matching decimal numbers and currency
      6m 45s
    7. Matching IP addresses
      7m 10s
    8. Matching dates
      7m 49s
    9. Matching times
      8m 59s
    10. Matching HTML tags
      8m 34s
    11. Matching passwords
      6m 49s
    12. Matching credit card numbers
      9m 36s
    13. Finding words near other words
      6m 38s
    14. Formatting with Search and Replace, pt. 1
      7m 22s
    15. Formatting with Search and Replace, pt. 2
      4m 15s
    16. Formatting with Search and Replace, pt. 3
      7m 10s
  12. 47s
    1. Goodbye
      47s

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