Simple. Tr's faster, because it doesn't have to do all the funky "regex" style matching. Tr just loops through the characters one-by-one, and if it finds a + it changes it to a space. If it doesn't, it doesn't spend any more time figuring out what not to do, whereas s/// will. In other words, s/// is like using a power drill when a screwdriver would do.

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Update: Okay, maybe it's not technically faster, but in theory it's supposed to be faster. I'm guessing that the s/// regex command gets more bit-level optimization than anything else in perl, just because it's such a swiss army knife of functionality; maybe tr got left behind on optimization?

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