Capturing

• \1
• $1
• ()
• capturing
• grouping

if ($row =~ /(apple|banana) pie/) {
    print $1;
}

Has a double character (e.g. 'oo' in loop)

# Input:   "my loop"

if ($line =~ /(.)\1/) {
    print $1;
}

Print if the same word appears twice in the same line

my $str = " in this line there is this word twice ";
if ($str =~ / ([a-z]+) .* \1 /) {
    print "$1\n";
}

/(.+).*\1/        # lines with anything more than once
/((.+).*\1)+/     # Syntax error ! Why ?

if ($line =~ /(.*)=(.*)/) {
    print "left:  $1\n";
    print "right: $2\n";
}