s/(            # begin capture
     \[error\]  # "error" in square brackets :)
     \s*        # optional whitespace
     (?:        # non capture
       \[       # literal [
       [^\]]+   # 1 or more non "]"
       \]       # literal ]
      )*        # end non capture 
    )           # end capture
   /$1<br>/x;