Easy-to-follow video tutorials help you learn software, creative, and business skills.Become a member

Escaping metacharacters

From: Using Regular Expressions

Video: Escaping metacharacters

Now that we've seen our first metacharacter, we've also seen one of the problems that comes up with metacharacters, which is, if a metacharacter has its special meaning most of the time, what happens when we want its literal meaning, or its alternate meaning? So in the case of the period that we saw with 9.00. we didn't want it to be a wildcard; what we wanted was for it to actually be a literal period, a decimal. Well, to do that, we use another metacharacter, which is the backslash character. What it says is escape the next character; give it a different meaning or an alternate meaning.

Escaping metacharacters

Now that we've seen our first metacharacter, we've also seen one of the problems that comes up with metacharacters, which is, if a metacharacter has its special meaning most of the time, what happens when we want its literal meaning, or its alternate meaning? So in the case of the period that we saw with 9.00. we didn't want it to be a wildcard; what we wanted was for it to actually be a literal period, a decimal. Well, to do that, we use another metacharacter, which is the backslash character. What it says is escape the next character; give it a different meaning or an alternate meaning.

So in the case of our period, if you wanted to match a literal period, or decimal, you would use a slash dot. So in our example with 9.00, you put the backslash in front of the dot/ Now we've escaped it. Now it matches 9.00, but it does not match 9500 or 9-00. See how that works? It's not just for the period; it's for any metacharacter that we have. If we even have a backslash and we want the literal backslash, well, then we escape it with a backslash. The first one has its metacharacter meaning saying to escape the next character. Then the next one doesn't have its metacharacter meaning anymore; it's its literal meaning, which is the actual backslash.

Now this is only for metacharacters that you're going to escape them. Literal characters should never be escaped, because in the case of literal characters a lot of times it gives them meaning. Backslash in front of a D means something different than a literal D. So be careful about escaping things that aren't metacharacters. An important point is that quotation marks are not metacharacters; they do not need to be escaped. Now if you're used to working in a programming language, you may be used to putting, let's say, double quotes around the outside of a string and then inside the string you need to escape those double quotes again, so you put a backslash in front of them.

It's a very common thing in programming. But in regular expressions, it is not a metacharacter; you do not need to escape them. All right! Let's try some of these out. Let's first try our example of 9.00, and let's put in 9.00, 9500, and 9-00. See, right now it matches all three of them. As soon as I escape that dot, now it only matches 9.00. See how that works? Let's try another one. Say we have two files--his_export.txt and her_export.txt--and what we want is to match both of them.

So we'd say h and then--so if we're looking at two of them, we'd think for this next character, we need a wildcard. There are other ways to do it, but for now we're going to use the wildcard. ..and then _export and then .txt. So you can do this and think, oh, wait a minute. This here I mean the wildcard; here I don't mean it. What I mean is the literal dot. See the difference? So here it's a wildcard, here it's not, and the highlighting helps give you that hint.

Now here, if something matched, it would still match. Let's try another one. I'll show you. Say we had resume1.txt, resume2.txt, and then the last one, let's say we have resume3_txt.zip. So let's say we're trying to write a regular expression that will match those. We want to match the first two, a .txt file, but not the third one. If we just try resume..txt, look what we've matched. We matched more than the dot.

What we want to do is escape the second one. The first one we want because we want it to pick up both the 1 and the 2 and to match both of those, but we don't wanted to match this third one, because it's not a.txt file. The last point I want to make about escaping is that you may need to also escape forward slashes as well. So here is /home/user/document.txt. That's a file path like we'd see from the command line, and if I wanted to write something that would match that, you might think, oh, well let's just basically write the same thing, but let's escape the dot.

You may also need to escape these, and the reason why is that right now I'm not typing the forward slash forward slash. Those forward slashes, remember I told you, in a lot of programming languages are the way that we denote a regular expression. If that's the case, well, then we're also going to need to escape these as well so that it knows that we're not done with our regular expression yet, right, because that second forward slash is the indicator that our regular expressions are all done. So just keep an eye on that. In this context we don't need to do it, but if we were using this inside something besides regexpal, and we had those forward slashes denoting our regex, well, then we would need to do it.

Escaping metacharacters, especially the wildcard metacharacter, is an important thing to remember. It seems simple, but it will trip you up if you're not careful.

Show transcript

This video is part of

Image for Using Regular Expressions
Using Regular Expressions

59 video lessons · 12274 viewers

Kevin Skoglund
Author

 
Expand all | Collapse all
  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

Start learning today

Get unlimited access to all courses for just $25/month.

Become a member
Sometimes @lynda teaches me how to use a program and sometimes Lynda.com changes my life forever. @JosefShutter
@lynda lynda.com is an absolute life saver when it comes to learning todays software. Definitely recommend it! #higherlearning @Michael_Caraway
@lynda The best thing online! Your database of courses is great! To the mark and very helpful. Thanks! @ru22more
Got to create something yesterday I never thought I could do. #thanks @lynda @Ngventurella
I really do love @lynda as a learning platform. Never stop learning and developing, it’s probably our greatest gift as a species! @soundslikedavid
@lynda just subscribed to lynda.com all I can say its brilliant join now trust me @ButchSamurai
@lynda is an awesome resource. The membership is priceless if you take advantage of it. @diabetic_techie
One of the best decision I made this year. Buy a 1yr subscription to @lynda @cybercaptive
guys lynda.com (@lynda) is the best. So far I’ve learned Java, principles of OO programming, and now learning about MS project @lucasmitchell
Signed back up to @lynda dot com. I’ve missed it!! Proper geeking out right now! #timetolearn #geek @JayGodbold
Share a link to this course

What are exercise files?

Exercise files are the same files the author uses in the course. Save time by downloading the author's files instead of setting up your own files, and learn by following along with the instructor.

Can I take this course without the exercise files?

Yes! If you decide you would like the exercise files later, you can upgrade to a premium account any time.

Become a member Download sample files See plans and pricing

Please wait... please wait ...
Upgrade to get access to exercise files.

Exercise files video

How to use exercise files.

Learn by watching, listening, and doing, Exercise files are the same files the author uses in the course, so you can download them and follow along Premium memberships include access to all exercise files in the library.


Exercise files

Exercise files video

How to use exercise files.

For additional information on downloading and using exercise files, watch our instructional video or read the instructions in the FAQ .

This course includes free exercise files, so you can practice while you watch the course. To access all the exercise files in our library, become a Premium Member.

Are you sure you want to mark all the videos in this course as unwatched?

This will not affect your course history, your reports, or your certificates of completion for this course.


Mark all as unwatched Cancel

Congratulations

You have completed Using Regular Expressions.

Return to your organization's learning portal to continue training, or close this page.


OK
Become a member to add this course to a playlist

Join today and get unlimited access to the entire library of video courses—and create as many playlists as you like.

Get started

Already a member ?

Become a member to like this course.

Join today and get unlimited access to the entire library of video courses.

Get started

Already a member?

Exercise files

Learn by watching, listening, and doing! Exercise files are the same files the author uses in the course, so you can download them and follow along. Exercise files are available with all Premium memberships. Learn more

Get started

Already a Premium member?

Exercise files video

How to use exercise files.

Ask a question

Thanks for contacting us.
You’ll hear from our Customer Service team within 24 hours.

Please enter the text shown below:

The classic layout automatically defaults to the latest Flash Player.

To choose a different player, hold the cursor over your name at the top right of any lynda.com page and choose Site preferences from the dropdown menu.

Continue to classic layout Stay on new layout
Exercise files

Access exercise files from a button right under the course name.

Mark videos as unwatched

Remove icons showing you already watched videos if you want to start over.

Control your viewing experience

Make the video wide, narrow, full-screen, or pop the player out of the page into its own window.

Interactive transcripts

Click on text in the transcript to jump to that spot in the video. As the video plays, the relevant spot in the transcript will be highlighted.

Learn more, save more. Upgrade today!

Get our Annual Premium Membership at our best savings yet.

Upgrade to our Annual Premium Membership today and get even more value from your lynda.com subscription:

“In a way, I feel like you are rooting for me. Like you are really invested in my experience, and want me to get as much out of these courses as possible this is the best place to start on your journey to learning new material.”— Nadine H.

Thanks for signing up.

We’ll send you a confirmation email shortly.


Sign up and receive emails about lynda.com and our online training library:

Here’s our privacy policy with more details about how we handle your information.

Keep up with news, tips, and latest courses with emails from lynda.com.

Sign up and receive emails about lynda.com and our online training library:

Here’s our privacy policy with more details about how we handle your information.

   
submit Lightbox submit clicked
Terms and conditions of use

We've updated our terms and conditions (now called terms of service).Go
Review and accept our updated terms of service.