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Line breaks and Multiline mode

From: Using Regular Expressions

Video: Line breaks and Multiline mode

In the last movie, we learned about the anchor metacharacters. And at the time, you may have noticed a small difference between the caret, and the backslash A, and between the dollar sign, and the backslash Z. They aren't exactly the same in their meaning because of how they handle new lines differently. In this movie, we'll learn why, and also learn how to apply these anchors to multi-line strings. First let's see the problem in action. So here in RegexPal, I just have a very simple shopping list that we've seen before, and I came up with the very simple, regular expression. Lowercase letters a to z, and a space inside a character set, repeated one or more times.

Line breaks and Multiline mode

In the last movie, we learned about the anchor metacharacters. And at the time, you may have noticed a small difference between the caret, and the backslash A, and between the dollar sign, and the backslash Z. They aren't exactly the same in their meaning because of how they handle new lines differently. In this movie, we'll learn why, and also learn how to apply these anchors to multi-line strings. First let's see the problem in action. So here in RegexPal, I just have a very simple shopping list that we've seen before, and I came up with the very simple, regular expression. Lowercase letters a to z, and a space inside a character set, repeated one or more times.

Now let's put an anchor at the beginning of this. Let's put the caret, and you can see that it matches, now, only milk. That's it. It didn't match each of those lines; it matched just the beginning of the entire string. Let's try it again, and at the end, let's put a dollar sign. Now it didn't match anything. What's going on there? Why didn't it match sweet potatoes? I have, actually, a line return after sweet potatoes, and if I take that line return away, now it matches. And that really is the secret to what the problem is here. It's because of those line returns.

When I put the line return character there, it then is an actual character, and the regex engine has to decide what to do with that character, and how it should handle it. We ran into the same problem with the dot. Should the dot match everything, including new line characters? We have that S option up here for that. It's a similar problem. There is an actual character that's invisible to us that goes right after each one of those lines, letting it know to put a line return in. So the regex engine is looking at this line for sweet potatoes, and it says, alright, I have O, E, S and now is the last thing here -- is this last thing after A to Z, and a space -- is it the end of the string? It's not.

I have got one more character there. That character is not the end of the string, so therefore I failed a match. It's only when we take that character away that it does match. This is called single-line mode, and by default, this is what regular expression engines use. They use single-line mode, and in that case, the caret and the dollar sign do not match at the line breaks. The same thing is true of the capital A and Z; they don't match at the line breaks. Many UNIX tools support only single-line. That's because in the early days when these tools were invented, they were invented as single-line tools.

And you'll remember that I told you that A and Z are not widely supported, and often can't be used in those UNIX tools as well. But over time, as things grew, single- line mode had to change, and we started using multi-line mode. People saw that there was a lot of utility in being able to deal with these lines as multiple lines. We can put our regular expression engine into multi-line mode. we will see how to do that in a moment. And once we do, suddenly the caret and the dollar sign will start matching at the start and the end of the lines instead, just like we might have expected them to do.

However, A to Z do not do that. A to Z always ignore the line breaks, it's counted as a character; A to Z just match the full string. Beginning of the document to the end of document; literally from A to Z. Most languages offer a multi-line option. Now, I don't know how to use even half of these languages. I may have a mistake up here. Definitely go to the language itself to make sure you have it exactly right. But in general, this is how you're going to put each one of these into multi-line mode. Typically, either you put an M after the regular expression to put it into multi-line mode, like you see there with the JavaScript example. Or, you pass in some kind of an option; some kind of a constant that tells it switch to multi-line when you process that, and that's what you see, like, with Java and .Net.

So let's try that multi-line option in our JavaScript tool. Let's go back, and let's just put our Caret at the beginning, and then multi-line anchors. It's the M option; that's what we want to pick: M option for multi-line anchors, and now you can see it does match each of those. Turn it off, turn it on, this matches only the Caret if it's at the beginning of the string, this matches it each and every time we get to a new line, and the same thing is true for the dollar sign. There we are; matches at the end, versus matches only the last item, and of course I can do the full string as well.

It works the exact same way. So that's it. It's a pretty simple thing to do once you have the concept, but it is an important concept. Otherwise, you may be thinking, well why in the world is this not matching? It ought to match it with the beginning and the end of line. Well, it's because you're not in multi-line mode. So just make sure that you take care when you're using those anchors to consider whether or not you're trying to match on those line returns, or whether you're trying to match the entire string all at once.

Show transcript

This video is part of

Image for Using Regular Expressions
Using Regular Expressions

59 video lessons · 11959 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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