$_ = 'ju[color=1]s[/color]t a sam[color=2]p[/color]le';

my $brackets = qr(\[[^\]]*\]); # text enclosed in brackets
my $char = qr([^\[\]\s]); # not spaces or brackets

s{
  (                # group to $1
    (?:
       $char          # a char
       (?:$brackets*) # followed by any number of brackets
    )
    {3}               # 3 times
  )
  (?:$brackets*)   # followed by any number of brackets

  (?=              # looking forward to ensure that
     $char         # we have at least one char
     [^\[\]]*      # and we are not inside of a brackets
     (?:
       \[
       |
       \z
     )
  )
}
{$1-}gx;

print;