If I put s/(?<!(?:href|src)=.{0,40})jpg//g in a perl file, and try to run it, it will give the warning: Variable length lookbehind is experimental in regex; marked by and fail.
But if put in a perl one-liner, it will run successfully, although still be warned of Variable length lookbehind is experimental in regex; marked by.
Is it by current design or maybe I'm using it wrong?
Update: I'm using perl 5.31.3
Before v5.30, a positive lookbehind with indeterminate, "variable width" pattern fails to compile with the Variable length lookbehind not implemented error.
In v5.30, you are allowed to use a lookbehind pattern that can match up to 255 chars.
Using a lookbehind assertion (like
(?<=foo?)or(?<!ba{1,9}r)previously would generate an error and refuse to compile. Now it compiles (if the maximum lookbehind is at most 255 characters), but raises a warning in the newexperimental::vlbwarnings category. This is to caution you that the precise behavior is subject to change based on feedback from use in the field.
If you use (?<=WORD\s+), you will get a Lookbehind longer than 255 not implemented error since the regex engine needs to know in advance that the length of the subpattern won't be longer than 255 and the + quantifier has an indeterminate length. So, (?<=WORD\s{0,255}) would work.
In your case, you know your lookbehind pattern will never match more than 255 chars, so just turn that experimental warning like any other experimental warnings:
no warnings qw(experimental::vlb);
Note: Make sure that the above line is placed after the use warnings; line, if present, or it will have no lasting effect, being overridden by use warnings;.