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Introduction to regular expressions

From: Unix for Mac OS X Users

Video: Introduction to regular expressions

In the previous movie we learned to use grep to search by using a simple text string as our search expression. Grep becomes even more powerful once we learn to use regular expressions. Now there is a lot to learn about regular expressions. We can have an entire course on them, and we won't too in-depth here, but in this movie we will cover the most essential concepts so that you can apply them and use them with grep. To begin with notice that in the Terminal I'm inside my user directory and I'll just change it to my unix_files directory. And the file that we're going to start by working with is the same fruit.txt file that we're working with in the last movie.

Introduction to regular expressions

In the previous movie we learned to use grep to search by using a simple text string as our search expression. Grep becomes even more powerful once we learn to use regular expressions. Now there is a lot to learn about regular expressions. We can have an entire course on them, and we won't too in-depth here, but in this movie we will cover the most essential concepts so that you can apply them and use them with grep. To begin with notice that in the Terminal I'm inside my user directory and I'll just change it to my unix_files directory. And the file that we're going to start by working with is the same fruit.txt file that we're working with in the last movie.

It's just a simple list of fruit that we can search inside of and see how regular expressions work. To begin with we saw that we can grep for apple inside fruit.txt. First thing I want to tell you about regular expressions is that it's a good idea to put quotes around it. Now they are not strictly required, you saw it worked just fine without them, but it's a good idea. And the reason why is because when we start working with regular expressions we're going to be using some special regular expression symbols. And some of those regular expression symbols also have a meaning to the Unix command line and by putting it in quotes we're helping the Unix command line to see ah, this is the expression.

The symbols that I see in there they are not for me to interpret and for me to do things with. They're meant to be part of the regular expression. So the single quotes are good habit to be in. Now the string that we've been searching for, apple, that is a regular expression. It's the simplest regular expressions that you can have, because it matches only the literal characters apple. Instead what we want to do is be able to provide some variation to that. So for example, we can replace those p's with periods. The period is a wild card character for regular expressions.

It says any character could match here. So now we're not just looking for apple we're looking for a followed by any character, followed by any other character, followed by l and e. It has to be 2 character. It can't be 1, but 2 characters, but we don't care what those characters are. So now it matches not just apple, but it matches ankle. Now it's not that useless in this case, because it really just does match apple just the same, but let's try another example. Let's do grep that file for .a.a.a inside fruit. Do you know what it's going to return? Now we get back both banana and papaya.

The reason why is because each of them matches the pattern that we've described. Any character followed by an a, any character followed by an a, and then any character followed by an a, and both of them match this pattern. And that's really what we're doing with regular expressions. We're defining patterns that we want to use for searching. So you can think of that. You can think of regular expressions as ways of defining patterns that we want to search for. Let's try another one let's just grep the file for e and a together, all right so, anytime that we have ea together. So it returned pear, peach, pineapple, and pear.

Now if we wanted to just get pear and peach, we can just put a p at the beginning, right? And that would rule out pineapple. That would no longer match our pattern, but if we wanted to get peach and pineapple and omit pear how we go about doing that? Well, we can say for example that we want the third character after the e and a to match either c or p, but not the r. Now you can see it returns peach and it returns pineapple, but it didn't return pear. We're going to talk more about the square brackets, but I want you to just understand the concept that what we're doing here is we're defining a pattern that we wanted to match and both eac and eap match our pattern.

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This video is part of

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Unix for Mac OS X Users

82 video lessons · 26046 viewers

Kevin Skoglund
Author

 
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  1. 3m 57s
    1. Introduction
      1m 14s
    2. Using the exercise files
      2m 43s
  2. 32m 2s
    1. What is Unix?
      7m 27s
    2. The terminal application
      4m 23s
    3. Logging in and using the command prompt
      5m 19s
    4. Command structure
      5m 22s
    5. Kernel and shells
      5m 25s
    6. Unix manual pages
      4m 6s
  3. 15m 58s
    1. The working directory
      2m 49s
    2. Listing files and directories
      3m 59s
    3. Moving around the filesystem
      4m 58s
    4. Filesystem organization
      4m 12s
  4. 1h 4m
    1. Naming files
      5m 41s
    2. Creating files
      2m 19s
    3. Unix text editors
      6m 39s
    4. Reading files
      5m 35s
    5. Reading portions of files
      3m 27s
    6. Creating directories
      2m 40s
    7. Moving and renaming files and directories
      8m 32s
    8. Copying files and directories
      3m 7s
    9. Deleting files and directories
      3m 38s
    10. Finder aliases in Unix
      4m 10s
    11. Hard links
      5m 30s
    12. Symbolic links
      6m 36s
    13. Searching for files and directories
      6m 32s
  5. 34m 58s
    1. Who am I?
      4m 3s
    2. Unix groups
      1m 52s
    3. File and directory ownership
      6m 41s
    4. File and directory permissions
      4m 27s
    5. Setting permissions using alpha notation
      6m 49s
    6. Setting permissions using octal notation
      3m 49s
    7. The root user
      1m 57s
    8. sudo and sudoers
      5m 20s
  6. 52m 34s
    1. Command basics
      4m 4s
    2. The PATH variable
      4m 13s
    3. System information commands
      3m 40s
    4. Disk information commands
      6m 8s
    5. Viewing processes
      5m 0s
    6. Monitoring processes
      3m 36s
    7. Stopping processes
      3m 19s
    8. Text file helpers
      6m 50s
    9. Utility programs
      7m 28s
    10. Using the command history
      8m 16s
  7. 20m 39s
    1. Standard input and standard output
      1m 24s
    2. Directing output to a file
      4m 13s
    3. Appending to a file
      2m 44s
    4. Directing input from a file
      5m 28s
    5. Piping output to input
      4m 40s
    6. Suppressing output
      2m 10s
  8. 41m 28s
    1. Profile, login, and resource files
      9m 11s
    2. Setting command aliases
      6m 59s
    3. Setting and exporting environment variables
      4m 54s
    4. Setting the PATH variable
      6m 10s
    5. Configuring history with variables
      6m 17s
    6. Customizing the command prompt
      6m 5s
    7. Logout file
      1m 52s
  9. 1h 25m
    1. grep: Searching for matching expressions
      5m 21s
    2. grep: Multiple files, other input
      4m 28s
    3. grep: Coloring matched text
      2m 57s
    4. Introduction to regular expressions
      3m 22s
    5. Regular expressions: Basic syntax
      3m 19s
    6. Using regular expressions with grep
      5m 20s
    7. tr: Translating characters
      8m 17s
    8. tr: Deleting and squeezing characters
      5m 30s
    9. sed: Stream editor
      7m 45s
    10. sed: Regular expressions and back-references
      7m 8s
    11. cut: Cutting select text portions
      7m 42s
    12. diff: Comparing files
      4m 35s
    13. diff: Alternative formats
      4m 30s
    14. xargs: Passing argument lists to commands
      7m 25s
    15. xargs: Usage examples
      7m 59s
  10. 42m 25s
    1. Finder integration
      4m 45s
    2. Clipboard integration
      5m 5s
    3. Screen capture
      3m 42s
    4. Shut down, reboot, and sleep
      3m 34s
    5. Text to speech
      2m 36s
    6. Spotlight integration: Searching metadata
      3m 41s
    7. Spotlight integration: Metadata attributes
      4m 24s
    8. Using AppleScript
      5m 23s
    9. System configurations: Viewing and setting
      5m 51s
    10. System configurations: Examples
      3m 24s
  11. 1m 26s
    1. Conclusion
      1m 26s

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