$text =~ s {(                  # Capture in $1
               <maths>         # Tag
               [^<]*           # Not a <, zero or more times
               (?:             # Group, non-capture.
                  <            # <,
                  (?!/maths>)  # not followed by /maths>
                  [^<]*        # Not a <, zero or more times
               )*              # Repeat.
               </maths>        # End tag.
             )                 # End capture $1.
             |           # Or
             &plus;            # Target.
           }
           {$1 || "&thinsp;&plus;&thinsp;"}exg;