s/(?:)// will do nothing (or rather, replace one-zero length string with another zero-length string). So, if you have a possibly-empty pattern in a variable, you could use 

s/(?:$pat)//

[download]

or 

my $re = qr/$pat/;

s/$re//

[download]

The latter was buggy before 5.18 as it resulted in the same as s/// when $pat contained an empty string.

In reply to Re^5: no way it's not a bug of Perl by ikegami
in thread I thought I found a bug of Perl by vincentaxhe

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