regex pod fixes related to /p as it stands today - #24554
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This documentation patch updates the docs to explain when and where /p is still relevent, and documents the performance reasons why. For a long time the docs have been incorrectly stating /p is not used or relevant anymore and that COW has made it redundant. This is unfortunately not the case, and it is still relevant for /g matches where the target string is not copied. Its a weird edge case, I bet no-one cares or actually needs it, but because of the flaws in our COW model we would make some code quadratic out of the box by making scalar /g matches copy the target string. As such we need an escape hatch for when the user wants to override that for some reason. I am inclined to think that removing PL_saw_ampersand was premature as well, COW isnt a complete solution for the performance issues that PL_saw_ampersand was solving.
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| They do not simply rely on copy-on-write for that, since copy-on-write | ||
| sharing is bounded and some target strings cannot use it. Use C</p> if | ||
| you need C<${^PREMATCH}>, C<${^MATCH}>, or C<${^POSTMATCH}> after such a | ||
| match. |
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I found "such" confusing. I think this means that if you need any one of the variables, add /p, but it kind of sounded like, "if you have bad experience, add /p"
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This is also related to #24147. |
book
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Jul 15, 2026
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Looks good to me. Nice to learn about the limitations of copy-on-write.
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@Leont: Since this is just a doc patch, why not ship it with 5.44? |
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This documentation patch updates the docs to explain when and where /p is still relevent, and documents the performance reasons why. For a long time the docs have been incorrectly stating /p is not used or relevant anymore and that COW has made it redundant. This is unfortunately not the case, and it is still relevant for /g matches where the target string is not copied. Its a weird edge case, I bet no-one cares or actually needs it, but because of the flaws in our COW model we would make some code quadratic out of the box by making scalar /g matches copy the target string. As such we need an escape hatch for when the user wants to override that for some reason.
I am inclined to think that removing PL_saw_ampersand was premature as well, COW isnt a complete solution for the performance issues that PL_saw_ampersand was solving.