Item 33. Watch out for the match variables

The match variables ($`, $&, and $') impose a speed penalty on your Perl programs because they cause Perl to perform some extra bookkeeping. After a successful match, they represent the part of the string that comes before the match, the part that matched, and the part that follows the match, respectively:

'My cat is Buster Bean' =~ m/Buster/;

print <<"HERE";
 Prematch: $`
    match: $&
Postmatch: $'
HERE