Programming Perl, 4th Edition

Chapter 5. Pattern Matching

Perl’s built-in support for pattern matching lets you search large amounts of data conveniently and efficiently. Whether you run a huge commercial portal site scanning every newsfeed in existence for interesting tidbits, a government organization dedicated to figuring out human demographics (or the human genome), or an educational institution just trying to get some dynamic information up on your website, Perl is the tool of choice, in part because of its database connections, but largely because of its pattern-matching capabilities. If you take “text” in the widest possible sense, perhaps 90% of what you do is 90% text processing. That’s really what Perl is all about and always has been about—in fact, it’s even part of Perl’s name: Practical Extraction and Report Language. Perl’s patterns provide a powerful way to scan through mountains of mere data and extract useful information from it.

You specify a pattern by creating a regular expression (or regex), and Perl’s regular expression engine (the “Engine”, for the rest of this chapter) then takes that expression and determines whether (and how) the pattern matches your data. While most of your data will probably be text strings, there’s nothing stopping you from using regexes to search and replace any byte sequence, even what you’d normally think of as “binary” data. To Perl, bytes are just characters that happen to have an ordinal value less than 256. (More on that in Chapter 6.)

If you’re acquainted with ...