Programming Perl, 4th Edition

Letting Perl Do the Work

When people of a certain temperament first learn regular expressions, they’re often tempted to see everything as a problem in pattern matching. And while that may even be true in the larger sense, pattern matching is about more than just evaluating regular expressions. It’s partly about looking for your car keys where you dropped them, not just under the streetlamp where you can see better. In real life, we all know that it’s a lot more efficient to look in the right places than the wrong ones.

Similarly, you should use Perl’s control flow to decide which patterns to execute and which ones to skip. A regular expression is pretty smart, but it’s smart like a horse. It can get distracted if it sees too much at once. So sometimes you have to put blinders onto it. For example, you’ll recall our earlier example of alternation:

/Gandalf|Saruman|Radagast/

That works as advertised, but not as well as it might, because it searches every position in the string for every name before it moves on to the next position. Astute readers of The Lord of the Rings will recall that, of the three wizards named above, Gandalf is mentioned much more frequently than Saruman, and Saruman is mentioned much more frequently than Radagast. So it’s generally more efficient to use Perl’s logical operators to do the alternation:

/Gandalf/ || /Saruman/ || /Radagast/

This is yet another way of defeating the “leftmost” policy of the Engine. It only searches for Saruman if Gandalf is nowhere to ...