Re: Driving the Engine right off the road

Lookahead/behind are zero-width assertions; you are asserting that after matching ^ (beginning of string), what comes just before that can't be XDG_, and then continuing to match the rest of your regex. There are two problems here: first, the beginning of the string will never follow XDG_, so your assertion will always pass; and second, you will only match if the string ends with _PATH or _PATH followed by a newline, which doesn't meet your requirements.

If perl supported variable length lookbehind, you could use a lookbehind at the end:

/^[_A-Z0-9]+$(?<!^XDG_.*_PATH)/
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but it doesn't. So use a lookahead instead, to test that the entire string is the expected characters but doesn't match your exclusion at the beginning:

/^(?!XDG_.*_PATH$)[_A-Z0-9]+$/
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or (and this is almost always the more readable approach), do it in code:

/^[_A-Z0-9]+$/ && ! /^XDG_.*_PATH$/
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Re^2: Driving the Engine right off the road by swl (Prior) on Aug 24, 2025 at 22:53 UTC
If perl supported variable length lookbehind, you could use a lookbehind at the end: Just a data point but Perl has limited variable length lookbehind as an experimental feature since version 5.30, with support for up to 255 characters. https://perldoc.perl.org/perlexperiment#(Limited)-Variable-length-look-behind	[reply]
Re^3: Driving the Engine right off the road by ysth (Canon) on Aug 25, 2025 at 22:29 UTC
See also https://perldoc.perl.org/perl5360delta#Variable-length-lookbehind-is-mostly-no-longer-considered-experimental I do wish it could work in even more cases, like where the beginning point is known (e.g. `(?<=\A...)` or with `\G`) or something like `(?<=...\1...)` where the length is known when running that part of the regex even if not when compiling it.	[reply] [d/l] [select]
Re^2: Driving the Engine right off the road by ysth (Canon) on Aug 24, 2025 at 14:58 UTC
Note that I kept your `$` but I hate it, because without `/m` it is rarely used to intentionally do what it actually does (match either the end of the string or before a newline at the end of a string). If your data has no newlines, use `\z` (match only at the end of the string). If your data ends with newlines, use `\n` (or `\n\z` if your string may have internal newlines that should not match). Then your code doesn't mislead about what the data looks like.	[reply] [d/l] [select]
Re^3: Driving the Engine right off the road by Intrepid (Curate) on Aug 24, 2025 at 21:55 UTC
It's weird. I lost some good habits when I took my 10-year break from programming, and using \z instead of $ in regexen is one of them. Since you pointed it out, I think my fingers will do the right thing from now on. In this case it wouldn't matter because my strings are all keys from `%ENV` and would never contain an embedded newline. But regaining (and retaining) good coding habits is important. – Soren never	[reply]