11.35. Replace spaces

s/^\s*//        leading
s/\s*$//        tailing

Better to write:
s/^\s+//        leading
s/\s+$//        tailing

both ends:

s/^\s*(.*)\s*$/$1/   " abc " =>  "abc "  because of the greediness

s/^\s*(.*?)\s*$/$1/   " abc " =>  "abc"  minimal match