Learning Perl, 5th Edition

More Powerful Regular Expressions

After already reading three (almost!) chapters about regular expressions, you know that they’re a powerful feature in the core of Perl. But there are even more features that the Perl developers have added; you’ll see some of the most important ones in this section. At the same time, you’ll see a little more about the internal operation of the regular expression engine.

Nongreedy Quantifiers

The four quantifiers you’ve already seen (in Chapters 7 and 8) are all greedy. That means that they match as much as they can, only to reluctantly give some back if that’s necessary to allow the overall pattern to succeed. Here’s an example: Suppose you’re using the pattern /fred.+barney/ on the string fred and barney went bowling last night. Of course, we know that the regular expression will match that string, but let’s see how it goes about it.^[*] First, of course, the subpattern fred matches the identical literal string. The next part of the pattern is the .+, which matches any character except newline, at least one time. But the plus quantifier is greedy; it prefers to match as much as possible. So it immediately matches all of the rest of the string, including the word night. (This may surprise you, but the story isn’t over yet.)

Now the subpattern barney would like to match, but it can’t—we’re at the end of the string. But since the .+ could still be successful even if it matched one fewer character, it reluctantly gives back the letter t at the end of the ...