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Answer: What can regular expressions NOT do?

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Q&A > regular expressions > What can regular expressions NOT do? contributed by I0

But Perl's m// operator is a more expressive grammar than formal regular expressions.
For example, \1 allows you to detect primes:
print "Prime" if (1 x shift) !~ /^1?$|^(11+?)\1+$/
And s/// can be used to detect balanced parenthesis:
$_ = '(()(()()))'; (my $re=$_)=~s/((\()|(\))|.)/${[')','']}[!$3]\Q$1\E${['(','']}[!$2]/gs +; eval{/$re/}; print "Balanced" unless $@ =~ /unmatched/;

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