my $winner = $1 if $R =~ /(\[[^#]*(?:\]|#[^#]*\]))/;
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As an explanation, that matches a literal left square bracket followed by anything that is not an octothorp, followed by either a right bracket or anything that is not an octothorpe then a right bracket. If you can have two "winning" groups on one line, you should use minimal rather than greedy matching.

Another way is:

my $winner = $1 if $R =~ /(\[[^#]*(?:#[^#]*)?\])/;
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Which means a literal left bracket followed by anything that is not an octothorpe followed by an optional octothorp and anything that is not an octothorp followed by a right bracket. I think I prefer this method. Here is a nicely commented version:

/(              # Start capturing.
    \[          # A literal left bracket.
    [^#]*       # Anthing that is not an octothorp
    (?:           # Group without capturing
       \#         # A literal octothorp
       [^#]*      # Anything that is not an octothorp
    )?            # End group. Group is optional.
    \]          # literal right bracket.
)/x
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-sauoq
"My two cents aren't worth a dime.";