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Regular expressions: Basic syntax

From: Unix for Mac OS X Users

Video: Regular expressions: Basic syntax

Now I'm not going to go through all of these item by item, but you can pause the movie if you want to take a look. Really get a feel for what they do. First a simple period, that's a wild card. The character set, so that's any character listed inside those square brackets. So in the example on the far right you can see that that would match both grey and gray, both spellings of gray could get matched. Then we have a negative character set. We'd have the same square brackets but we put a caret as the first character inside of them and that would say any character that's not included there. So for example, if we wanted to match everything that was not an A, E, I, O or U, then we would use that negative character set.

Regular expressions: Basic syntax

Now I'm not going to go through all of these item by item, but you can pause the movie if you want to take a look. Really get a feel for what they do. First a simple period, that's a wild card. The character set, so that's any character listed inside those square brackets. So in the example on the far right you can see that that would match both grey and gray, both spellings of gray could get matched. Then we have a negative character set. We'd have the same square brackets but we put a caret as the first character inside of them and that would say any character that's not included there. So for example, if we wanted to match everything that was not an A, E, I, O or U, then we would use that negative character set.

Next we have the range indicator which is a minus sign and that's typically just used inside a character set to indicate a range of characters. You don't have to actually spell out every single character you mean. You can just say capital A to capital Z, A dash Z, lowercase a to z, 0 to 9, and those are the typically the cases you would use them with is with the alphanumeric characters and with digits. The next three items the asterisk, the plus, and the question mark all have to do with how often element repeats. Does it occur zero or more times, one or more time, or zero or one time? So to take the first one as an example, file_name, that underscore may or may not exist.

If it's not there it still matches. If it exist one time it still matches. If it exists two times, three times, it doesn't matter. It still matches. Below those we have the alternation operator. This is the upright pipe that we've already seen we were piping commands. Notice that this is one of the symbols that has a meaning to Unix that's different than the meaning in regular expressions. Here it's the OR operator. So what it means is for example I asked for JPG or GIF or PNG. That's very common if you want all three file types to be matched.

Next we have the start and end of line anchors. The start of line anchor is the caret symbol. That's the same thing we used up on the negative character set, but it's in a different context here. Don't get them confused. If it's inside the square brackets, it's the negative characters set. Otherwise, we're really talking about this start of line anchor and that means that a line beginning with hello would match, but if hello was somewhere in the line, not at the beginning, it would not match. Same thing for the end of line anchor. If you need an actual literal character and you want to escape one of these regular expression characters, you would use the backslash and then there's a number of them at the bottom that I won't go over in detail, but they match either any digit, anything not a digit, anything that's an alphanumeric character or not, anything that's white space or not and so on.

Notice though that three of those I've marked with an asterisk. These are extended regular expression syntax. We'll talk more about that. But for now, just notice that there are a couple of these that are part of the extended set and some of them that are part of the basic set. There're also some predefined character classes that can help you to find certain types of things easier. The symbols for these are a little bit longer. All of them have square brackets and colons on either side of a keyword. So for example, alpha would match any of those alphabetic characters. Digit would match any numeric characters.

Alnum would match both, alphabetic or numeric, and so on. Now you could just as easily use those other symbols to write these out yourself. These just provide useful shortcuts. Notice here that the square bracket is not indicating a character set. This is talking about a single character. If you want to say this is the character set, we put another set of square brackets around that.

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This video is part of

Image for Unix for Mac OS X Users
Unix for Mac OS X Users

82 video lessons · 25577 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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