/literal/ or /other-literal/ is going to be faster than /literal|other-literal/ because the engine can use the Boyer-Moore optimization to search the string. You'll want to look at how much it's able to skip when guessing the start of the match.

Separately, in perl <5.10, the time required to fail an alternation scales linearly with the size of the regexp. This is still true with your explicit /foo/ or /bar/. In 5.10, the alternation uses a trie and it's clearly a winner. I'd like to be able to tell you how it scales but it's less than O(N). To be future compatible, you'll prefer alternation when combining lists of literals and in some cases for some non-literals (I can't enumerate those for you).

The interpolation of variables is a red herring and should be costed separately. See /o is dead, long live qr//! for the scoop on that.

Further, this is much easier to reason about if you forget the idea of benchmarking this stuff. It's all fast enough that you'll need to be especially careful to promote the testable costs above the noise. If you have knowledge of the optimizations in use in each case you can potentially design your tests around things that use them and not. I

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In reply to Re: Benchmarking regex alternation by diotalevi
in thread Benchmarking regex alternation by Tanktalus

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