July 2012
Beginner
151 pages
3h 27m
English
Content preview from Introducing Regular ExpressionsBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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Chapter 4. Alternation, Groups, and Backreferences
You have already seen groups in action. Groups surround text with parentheses to help perform some operation, such as the following:
Performing alternation, a choice between two or more optional patterns
Creating subpatterns
Capturing a group to later reference with a backreference
Applying an operation to a grouped pattern, such as a quantifer
Using non-capturing groups
Atomic grouping (advanced)
We’ll be using a few contrived examples, in addition to the text from “The Rime of the Ancyent Mariner” again, in rime.txt. This time, I’ll use the desktop version of RegExr, as well as other tools like sed. You can download the desktop version of RegExr from http://www.regexr.com, for Windows, Mac, or Linux (it was written with Adobe AIR). Click the Desktop Version link on the RegExr web page (lower-right corner) for more information.
Alternation
Simply said, alternation gives you a choice of alternate patterns to match. For example, let’s say you wanted to find out how many occurrences of the article the are in the “The Rime of the Ancient Mariner.” The problem is, the word occurs as THE, The, and the in the poem. You can use alternation to deal with this peculiarity.
Open the RegExr desktop application by double-clicking on its icon. It looks very much like the online version but has the advantage of being local on your machine, so you won’t suffer the network issues that sometimes occur when using web applications. I’ve copied and pasted the ...
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