Variable-Length Lookbehind not work in perl file, but work in one-liner

Question

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 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 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

Answer 1

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 new experimental::vlb warnings 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; .

Answer 2

Perl doesn't care if the code is provided via -e or via a file. The code behaves identically in both situations. The reason it fails is not because you used the code you posted in a file instead of on the command line. There is another difference you haven't identified causing the difference in behaviour.

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That said you shouldn't use the cost you posted. As the warning loudly announces, this is an experimental feature . It is unproven, and thus potentially problematic. Also, it is subject to change and removal without further notice.

You can eliminate the use of the experimental variable-length lookbehind by using the following instead:

s/(?:href|src).{0,40}(*SKIP)(*FAIL)|jpg//g

(*FAIL) causes the pattern not to match, and thus trigger backtracking. But (*SKIP) made it so that it will start matching at the position after the string matched by (?:href|src).{0,40} in future attempts.