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Using regular expressions with grep

From: Unix for Mac OS X Users

Video: Using regular expressions with grep

So we've already gotten a look at the wildcard, the period, and we've also seen character sets when we were working with finding both peach and pineapple. Let's try the beginning Of line and end Of line anchors. So for example grep and the beginning of line anchor followed by P inside fruit will find every line that begins with P. Notice it did not match apple. Apple has a P in it, but it's not at the beginning of the line. I am going to do the same thing. We can find everything with berry at the end and that will return every line that has berry at the end.

Using regular expressions with grep

So we've already gotten a look at the wildcard, the period, and we've also seen character sets when we were working with finding both peach and pineapple. Let's try the beginning Of line and end Of line anchors. So for example grep and the beginning of line anchor followed by P inside fruit will find every line that begins with P. Notice it did not match apple. Apple has a P in it, but it's not at the beginning of the line. I am going to do the same thing. We can find everything with berry at the end and that will return every line that has berry at the end.

Now in this case actually all occurrences of berry were also at the end of the line, but it would find it if it were only at the end of the line. Let me show you that. One way that you can work with grep that's really useful is instead of working with a file, let's say we have berry bush, we can use pipes from echo, so we'll pipe-in the string, and then we'll grep for 'berry$'. Now, it didn't find it, because it's not at the end of the line. If we instead we're looking for berry bush berry, now it does find it. It matched the last one, not the first one.

You can see I have my colorization turned on, so it just colorized the last one. If we did the same thing but we take away that end Of line anchor, now it finds it both times. Let's try another one. Let's do echo, and let's do AaBbCcDdEe. There we go! And let's pipe that into grep, and this time I'm going to make sure that color is on. If you don't have your color on, this will turn your color on. Let's search for just upper. We're looking for uppercase. We're going to use that character class.

Notice what happened here. It didn't match the uppercase letter that you might have expected it to, because it's not interpreting this as being a character class. It's interpreting this as being a character set. It thinks that we want to match anything that is in this character set. So E is in this character set, so it got matched. There is no U, P, and R. That's why the lowercase E got matched. If we instead put double-brackets around this, and we actually should probably put quotes around it too. It's always a good practice, remember.

Now, it returns what we expect it to match. So I just point that out to you to make sure that you see with the single brackets we're actually referring to the class of characters. But if we wanted to actually work inside grep, we use the double ones so that it's saying a character set made up of the character class. I'm just going to paste-in another example, so you don't have to watch me type it. I've just got a bunch of punctuation here and I'm going to search that to find all the punctuation. I think that gives you the basics for regular expressions. Again, it's a very deep subject, and this is really only the surface of it.

There are even sites that catalog regular expressions. So if you're looking for a regular expression that will match every phone number or every email address, all those different combinations for how those might be formatted, those exist. People have written them and they've shared them on different web sites and you can make use of them without having to reinvent the wheel. But what I do want to show you is there are a couple of things to watch out for when using regular expressions with grep. The first of these is that if we were to grep for 'ap*le' inside *fruit.txt, that the two asterisks here have different meanings.

This is the regular expression asterisk. It means that the P is repeated zero or more times. This would match ALE, APLE, APPLE, APPLE, and so on. This one is a wildcard for the file system, totally different meaning. So I want to make sure that you see those and realize the difference. That's part of why we want to make sure that we put these inside quotes, is to help keep that separate. The second is I'll take away the asterisk here and I'll run the command and you'll see that it does return the results that we would expect. If we run this other version with the plus sign instead, that's the operator that means one or more times.

That would match APLE and APPLE, but not ALE. It has to occur at least once. That's what the plus symbol means. If we run it now, we get nothing back. The reason why is it's taking that plus to be the literal plus sign. This is what I was talking about with the basic and extended regular expressions. So it's an important sort of gotcha when working with regular expressions, that there are the basic ones that work all the time, and then there is this extended set of few things that only work in some cases. If we want to use the extended set in grep, we need to use the -E option. Now it works! Now, it finds it exactly as we would expect.

So if you want to use a few of these extra features, you're going to need to use that -E. There are more of them than just the three I showed you. Those are the most basic and the ones that come up and cause the problem most often. Could you use -E all the time? Absolutely. Another spot where this basic and extended regular expressions causes problems is when we're working with that OR operator as well. So for example, if we grep for Apple or Pear inside fruit.txt, so it feels like a very simple operator. It feels like something we ought to be able to do. It says oops, it's not there.

All we've got to do is put that -E in front of it and now it does find every occurrence of either Apple or pear. So regular expressions, especially when combined with rep, is really great for finding exactly what you want. But what it doesn't do is it doesn't allow you to change or manipulate anything. All it does is allow you to find it. So in order to make changes to manipulate the data, we're going to need to take a look at a few more Unix tools.

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This video is part of

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Unix for Mac OS X Users

82 video lessons · 26200 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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