bm.md

str = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaabbb" 

1.9.3p0 :009 > Benchmark.measure { 1_000_000.times { str.sub(/b+\z/, "") } }
 =>   1.410000   0.010000   1.420000 (  1.410129)
 
1.9.3p0 :010 > Benchmark.measure { 1_000_000.times { str.sub(/b+\z/, "") } }
 =>   1.400000   0.000000   1.400000 (  1.405524)
 
1.9.3p0 :011 > Benchmark.measure { 1_000_000.times { str.sub(/b+\z/, "") } }
 =>   1.400000   0.000000   1.400000 (  1.396972)
 

1.9.3p0 :012 > Benchmark.measure { 1_000_000.times { str.gsub(/b+\z/, "") } }
 =>   3.130000   0.020000   3.150000 (  3.148422)
 
1.9.3p0 :013 > Benchmark.measure { 1_000_000.times { str.gsub(/b+\z/, "") } }
 =>   3.140000   0.030000   3.170000 (  3.168520)
 
1.9.3p0 :014 > Benchmark.measure { 1_000_000.times { str.gsub(/b+\z/, "") } }
 =>   3.130000   0.020000   3.150000 (  3.149087)
 

1.9.3p0 :015 > Benchmark.measure { 1_000_000.times { str.sub(/b*\z/, "") } }
 =>   2.530000   0.000000   2.530000 (  2.540740)
 
1.9.3p0 :016 > Benchmark.measure { 1_000_000.times { str.sub(/b*\z/, "") } }
 =>   2.540000   0.000000   2.540000 (  2.533296)
 
1.9.3p0 :017 > Benchmark.measure { 1_000_000.times { str.sub(/b*\z/, "") } }
 =>   2.570000   0.000000   2.570000 (  2.571964)