It would probably be better to link to the article itself. Anyway, I get the idea that (?~abc) would be analogous to [^x], except that the former is for multi-character sequences. I assume that similar effects can be achieved in Perl with Lookaround Assertions. What I'm missing at the moment are some more examples of practical applications - the only one mentioned in the article and the Ruby docs is not matching invalid C comments (second final example below).

An excerpt from the Ruby docs:

(?~subexp) absent operator (experimental)

Matches any string which doesn't contain any string which matches subexp. Similar to (?:(?!subexp).)*, but easy to write.

Unlike (?:(?!abc).)*c, (?~abc)c matches "abc", because (?~abc) matches "ab".

A sandbox for finding equivalent expressions:

use warnings;
use strict;
use Test::More;

# in Ruby: (?~abc)
my $re1 = qr{ \A (?: (?!abc) . )* \z }x;
like '', $re1;
like 'ab', $re1;
like 'aab', $re1;
like 'ccdd', $re1;
unlike 'abc', $re1;
unlike 'aabc', $re1;
unlike 'ccccabc', $re1;
unlike 'ccabcdd', $re1;

# in Ruby: (?~abc)c
# this example fails in Perl
like 'abc', qr{ \A (?: (?!abc) . )* c \z }x;

# in Ruby: \A\/\*(?~\*\/)\*\/\z
my $re2 = qr{ \A /\* ( (?!\*/). )*? \*/ \z }x;
like '/**/', $re2;
like '/* foobar */', $re2;
unlike '/**/ */', $re2;

done_testing;

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In reply to Re: [OT] Thoughts on Ruby's new absent operator? by haukex
in thread [OT] Thoughts on Ruby's new absent operator? by perlancar

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