s/($NOCHANGE)|$TOCHANGE/defined $1 ? $1 : $REPLACE /ge;

##</code><code>##

s/($NOCHANGE)|$TOCHANGE/$1 || $REPLACE /ge;

##</code><code>##

$text =~ s<(<maths>.*?</maths>)|&plus;>{ $1 || '&thinsp;&plus;&thinsp;' }ge;