Using Regular Expressions
Illustration by Mark Todd

Alternation metacharacter


From:

Using Regular Expressions

with Kevin Skoglund

Video: Alternation metacharacter

In the last movie we learned about the grouping metacharacters, and those are going to be really useful as we turn our attention now to talk about alternation. The alternation metacharacter is just a single upright line. Take a second to locate that on your keyboard. On most keyboards it's going to be above the Return key. This is often referred to as the pipe character, because in UNIX its function is to pipe output from one command to another. In regular expressions, you'll usually hear me read it as "or," but if you hear me say pipe or pipe character, that's what it means--even though it serves a different purpose in regular expressions than it does in UNIX.
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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 27s
    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 31s
    6. POSIX bracket expressions
      4m 13s
  5. 36m 39s
    1. Repetition metacharacters
      7m 17s
    2. Quantified repetition
      6m 59s
    3. Greedy expressions
      6m 27s
    4. Lazy expressions
      6m 47s
    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 32s
    1. Positive lookahead assertions
      6m 39s
    2. Double-testing with lookahead assertions
      7m 16s
    3. Negative lookahead assertions
      6m 11s
    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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Watch the Online Video Course Using Regular Expressions
5h 36m Intermediate Nov 21, 2011

Viewers: in countries Watching now:

Learn how to find and manipulate text quickly and easily using regular expressions. Author Kevin Skoglund covers the basic syntax of regular expressions, shows how to create flexible matching patterns, and demonstrates how the regular expression engine parses text to find matches. The course also covers referring back to previous matches with backreferences and creating complex matching patterns with lookaround assertions, and explores the most common applications of regular expressions.

Topics include:
  • Creating flexible patterns using character sets
  • Achieving efficiency when using repetition
  • Understanding different types of search strategies
  • Writing logical and efficient alternations
  • Capturing groups and reusing them with backreferences
  • Developing complex patterns with lookaround assertions
  • Working with Unicode and multibyte characters
  • Matching email addresses, URLs, dates, HTML tags, and credit card numbers
  • Using search and replace to format a document
Subject:
Developer
Software:
Regular Expressions
Author:
Kevin Skoglund

Alternation metacharacter

In the last movie we learned about the grouping metacharacters, and those are going to be really useful as we turn our attention now to talk about alternation. The alternation metacharacter is just a single upright line. Take a second to locate that on your keyboard. On most keyboards it's going to be above the Return key. This is often referred to as the pipe character, because in UNIX its function is to pipe output from one command to another. In regular expressions, you'll usually hear me read it as "or," but if you hear me say pipe or pipe character, that's what it means--even though it serves a different purpose in regular expressions than it does in UNIX.

Now in some programming languages they use double pipes to represent "or." Be careful if you're familiar with those that you only use a single pipe here. So as I said, the pipe character is an or operator. It means either match the expression on the left or match the expression on the right, and it's ordered; it does them in that order: the leftmost first, then the one on the right. And you could have more than just two. You can daisy-chain them together, so we can have option one or option two or option three and so on. We can also use those grouping metacharacters that we just learned to group our alterations to keep them distinct from the rest of our expression.

I think that's a really good practice. Most of the time you're going to want to do that so it's really clear where the alternation stops and ends, because it's got those parentheses around it. It's also going to be important in some cases, because it changes the meaning if we don't have them. We'll see an example of that. Let's start by looking at some simple examples, if we just have apple or orange. So it matches the literal characters a-p-p-l-e or the literal characters o-r-a-n-g-e. Notice that those two strings do not have the same length. The parser is able to handle it, the fact that they're two different lengths.

It looks for either the word apple and if it can't find that, then it backtracks and looks for the word orange. If we have them daisy-chained together-- let's say we have abc|def|ghi|jkl--then it will any of those four combinations. As I was saying about putting the grouping operators around it to make it clear, apple followed immediately by either juice or sauce is not the same thing as apple juice or sauce--two separate things. One of the simplest use cases that you can use this alternation for is to find misspelled words.

So if you wanted to search a document for weird and you wanted to find it both spelled correctly and incorrectly, well, you could put inside your group expression an alternation ei or ie. Let's try these out. So to start with, I'm just going to paste in some text here, just so you can see. I've got apple and orange, appleorange all run together, an apple with an upright pipe orange, and that's my string. Let's also use apple|orange as a regular expression, and you can see what it matches. It matches apple or orange. And appleorange, it matched but only because it matched the two parts of that, apple and orange.

It didn't match the whole world together and even putting the pipe in the middle. I just want you to see that that operator is doing something that's not the literal character. If we put the escape in front of it, of course then it matches the literal version. Let's try another example. Let's just put the alphabet in here, and let's look for abc|def|ghi|jkl. See, it matches each one of those. Now we've got Global turned on. If I turn that off, you see it just matches the first one. It started at the beginning, and it said, all right, do I see an abc. Oh! I do. Great! I'm all done. If it hadn't found it, if let's say this had been acc, we'll, then it would try again.

It will keep moving until it got to the def. So it just moves along each one. It says, did this one succeed? No, it did not. Did this one succeed? Yes, it did. So now I have a match, and we never execute what's after that. We never even bother looking at it. We've got a match, and we can move on. Now as I said, grouping is always a good idea. Let's try our apple juice and apple sauce example and apple, try it this way first, juice or sauce. So it's either juice or sauce. Let's turn back on Global so that you can see it.

It found the word sauce because we said sauce was a valid choice, but it did not find applesauce. If we put the parentheses around it, now it finds what we intended, apple immediately followed by either our two expressions: either juice or sauce. Now, I've been using just literal text characters in each of these expressions here, just to make it simple so you can see, but these can be anything you want. These can be absolutely any expression. They can be expressions that include more alternations. They have character sets, repetition--all that stuff is allowable here.

The main principle here is just that it's expression one or it is expression two. Last of all, let's just try those misspelled words. Let's say that we have weird or wierd and then up here w(ei|ie)rd. It matches both possibilities. So that's the fundamentals of working with alternations. In the next movie, I want us to go a little bit deeper and talk about the way that the regular expression engine parses through these and also make sure that we know how to write good logical alternations.

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