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s/         #replace
^[\s]*     #the start of the string followed by any number of spaces
...
(?<!\s)    #look-behind assertion to get to the last non-whitespace ch
+ar
\s*$       #match whitespace until EOL
/$1/x;     #replace with the (.*)

Benchmark: timing 100000 iterations of my_long_one, one_liner, two_lin
+er...
my_long_one:  6 wallclock secs ( 4.62 usr +  0.00 sys =  4.62 CPU) @ 2
+1659.09/s (n=100000)
one_liner:  3 wallclock secs ( 3.89 usr +  0.00 sys =  3.89 CPU) @ 256
+73.94/s (n=100000)
...
my_long_one => s/^[\s]*(.*)(?<!\s)\s*$/$1/
one_liner => s/^\s+|\s+$//g
two_liner => s/^\s+//g; s/s+$//g