Chapter 3. Overview of Regular Expression Features and Flavors

Now that you have a feel for regular expressions and a few diverse tools that use them, you might think we're ready to dive into using them wherever they're found. But even a simple comparison among the egrep versions of the first chapter and the Perl and Java in the previous chapter shows that regular expressions and the way they're used can vary wildly from tool to tool.

When looking at regular expressions in the context of their host language or tool, there are three broad issues to consider:

• What metacharacters are supported, and their meaning. Often called the regex "flavor."

• How regular expressions "interface" with the language or tool, such as how to specify regular-expression operations, what operations are allowed, and what text they operate on.

• How the regular-expression engine actually goes about applying a regular expression to some text. The method that the language or tool designer uses to implement the regular-expression engine has a strong influence on the results one might expect from any given regular expression.

Regular Expressions and Cars

The considerations just listed parallel the way one might think while shopping for a car. With regular expressions, the metacharacters are the first thing you notice, just as with a car it's the body shape, shine, and nifty features like a CD player and leather seats. These are the types of things you'll find splashed across the pages of a glossy brochure, and a list of metacharacters like the one in Section 1.5.6 is the regular-expression equivalent. It's important information, but only part of the story.

How regular expressions interface with their host program is also important. The interface is partly cosmetic, as in the syntax of how to actually provide a regular expression to the program. Other parts of the interface are more functional, defining what operations are supported, and how convenient they are to use. In our car comparison, this would be how the car "interfaces" with us and our lives. Some issues might be cosmetic, such as what side of the car you put gas in, or whether the windows are powered. Others might be a bit more important, such as if it has an automatic or manual transmission. Still others deal with functionality: can you fit the thing in your garage? Can you transport a king-size mattress? Skis? Five adults? (And how easy is it for those five adults to get in and out of the car—easier with four doors than with two.) Many of these issues are also mentioned in the glossy brochure, although you might have to read the small print in the back to get all the details.

The final concern is about the engine, and how it goes about its work to turn the wheels. Here is where the analogy ends, because with cars, people tend to understand at least the minimum required about an engine to use it well: if it's a gasoline engine, they won't put diesel fuel into it. And if it has a manual transmission, they won't forget to use the clutch. But, in the regular-expression world, even the most minute details about how the regex engine goes about its work, and how that influences how expressions should be crafted and used, are usually absent from the documentation. However, these details are so important to the practical use of regular expressions that the entire next chapter is devoted to it.

In This Chapter

As the title might suggest, this chapter provides an overview of regular expression features and flavors. It looks at the types of metacharacters commonly available, and some of the ways regular expressions interface with the tools they're part of. These are the first two points mentioned at the chapter's opening. The third point —how a regex engine goes about its work, and what that means to us in a practical sense—is covered in the next few chapters.

One thing I should say about this chapter is that it does not try to provide a reference for any particular tool's regex features, nor does it teach how to use regexes in any of the various tools and languages mentioned as examples. Rather, it attempts to provide a global perspective on regular expressions and the tools that implement them. If you lived in a cave using only one particular tool, you could live your life without caring about how other tools (or other versions of the same tool) might act differently. Since that's not the case, knowing something about your utility's computational pedigree adds interesting and valuable insight.