Using Regular Expressions
Illustration by Mark Todd

Using Regular Expressions

with Kevin Skoglund

Video: Repeating and nesting alternations

In this movie, I want us to just take a look at repeating and nesting our alternations. You can repeat your alternations. It's just simply a group, and that group can be repeated, just like we were doing grouped expressions before and then putting repetitions on them. Keep in mind though that with the nested alternations that the first matched alternation does not have any effect on the future matches. So if we have a expression where AA is the first choice, BB is the second choice, and CC is the third choice, if we repeat it six times, if we have a string where the first thing that matches is AA, that does not mean that AA is the thing that has to be repeated over and over.
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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

Repeating and nesting alternations

In this movie, I want us to just take a look at repeating and nesting our alternations. You can repeat your alternations. It's just simply a group, and that group can be repeated, just like we were doing grouped expressions before and then putting repetitions on them. Keep in mind though that with the nested alternations that the first matched alternation does not have any effect on the future matches. So if we have a expression where AA is the first choice, BB is the second choice, and CC is the third choice, if we repeat it six times, if we have a string where the first thing that matches is AA, that does not mean that AA is the thing that has to be repeated over and over.

It's the same way as when we repeated something of the digit class. We weren't saying that if we match the digit 9 and we were looking for four of those, then we had to have 9 four times. It's the digit class that gets repeated four times. So here's the whole alternation that gets repeated four times. And of course, we can nest alternations as well, but you just need to be careful with it. First you just want to make sure to check your nesting carefully to make sure that it does logically make sense. It can get a little bit tricky. If you have a really long regular expression and you've got lots of square brackets and parentheses and curly braces in there, backslashes in front of those shorthand character sets, it makes a lot harder to see where the alternations are and make sure that it does parse carefully.

One good tip is to put these into a text document and put line returns between them to start with. Make sure that each one of the parts works out the way you think it should, and then you can remove the line returns at the end so that it becomes one long line again. Overall with nesting it becomes a trade-off between precision, readability, and efficiency. Remember the things we talked about. With efficiency working with alternations, that effect becomes compounded when we start nesting them inside each other. Let's go to regexpal and see. So to start with, let's just have a simple regular expression here, and we're going to say the regular expression is either going to be two digits or it's going to be the characters A to Z or A to Z again--two uppercase letters--and that whole thing can repeat three times.

That's going to match 112233 just fine, and it's also going to match AABBCC just fine. Match AA66ZZ or 11AA44. It doesn't have to be the same thing each time as it repeats. It's the alternation, a whole alternation expression, that's repeated. So it's a lot to fluctuate between which one matches: option one or option two. Now for nesting, I'm just going to put in sort of a grocery list here of some random items, and then let's paste in a long nested expression here.

So I've got apple followed by either juice or sauce or milk, which optionally might have shake after it, or sweet, space, followed by either peas, corn, or potatoes. So that's the regular expression. It matches everything on my shopping list except for yogurt. Now it's really a matter of taste and personal preference whether you find this simpler or if you find this simpler. Just simple, non-nested ors. I've got milk and milk shake written as two things, sweet peas, sweet corn, sweet potatoes. Each time I just write them out again.

Or you may decide that you like it somewhere in between. Maybe you want to make milk shake and have shake be optional. It really is about your personal taste and also about what you're trying to accomplish. You can see here it didn't match yogurt on my shopping list. Well, maybe that was intentional; maybe your purpose was to try and find all these other items and not some items, like yoghurt. But if what you're really trying to find was all of the words and spaces that might make up a word and you wanted to find it that way, well that's a whole lot simpler. So it really just depends on your use case. But just be careful when you're working with those nested alternations that you don't either confuse yourself or come up with some very elaborate and inefficient expression that takes forever to run.

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