Mastering Regular Expressions, 3rd Edition

Perl Efficiency Issues

For the most part, efficiency with Perl regular expressions is achieved in the same way as with any tool that uses a Traditional NFA. Use the techniques discussed in Chapter 6 — the internal optimizations, the unrolling methods, the “Think” section — all apply to Perl.

There are, of course, Perl-specific issues as well, and in this section, we’ll look at the following topics:

• There’s More Than One Way To Do It  Perl is a toolbox offering many approaches to a solution. Knowing which problems are nails comes with understanding The Perl Way, and knowing which hammer to use for any particular nail goes a long way toward making more efficient and more understandable programs. Sometimes efficiency and understandability seem to be mutually exclusive, but a better understanding allows you to make better choices.
• Regex Compilation, qr/···/, the /o Modifier, and Efficiency  The interpolation and compilation of regex operands are fertile ground for saving time. The /o modifier, which I haven’t discussed much yet, along with regex objects (qr/···/), gives you some control over when the costly re-compilation takes place.
• The $& Penalty  The three match side effect variables, $', $&, and $', can be convenient, but there’s a hidden efficiency gotcha waiting in store for any script that uses them, even once, anywhere. Heck, you don’t even have to use them — the entire script is penalized if one of these variables even appears in the script.
• The Study Function  Since ages ...