Ruby Benchmark: What's the fastest way to check the start of a String in Ruby?

benchmark.rb

require 'benchmark'

TIMES = 1000000

STRING = "this is a long and bad-ass string"

Benchmark.bmbm do |test|
  test.report("Index") do
    
    TIMES.times { STRING.index("this is") }
  end

  test.report("start_with?") do
    TIMES.times { STRING.start_with?("this is") }
  end
  
  test.report("=~ (^)") do
    TIMES.times { STRING =~ /^this is/ }
  end

  test.report("=~ (\A)") do
    TIMES.times { STRING =~ /^this is/ }
  end
  
  test.report("scan (^)") do
    TIMES.times { STRING.scan /^this is/ }
  end


  test.report("scan (\A)") do
    TIMES.times { STRING.scan /^this is/ }
  end
  
end

results.txt

Rehearsal ------------------------------------------------
Index          0.260000   0.000000   0.260000 (  0.264949)
start_with?    0.190000   0.010000   0.200000 (  0.193379)
=~ (^)         0.270000   0.000000   0.270000 (  0.272468)
=~ (A)         0.250000   0.000000   0.250000 (  0.258506)
scan (^)       1.740000   0.020000   1.760000 (  1.776399)
scan (A)       1.750000   0.010000   1.760000 (  1.776322)
--------------------------------------- total: 4.500000sec

                  user     system      total        real
Index          0.280000   0.000000   0.280000 (  0.283709)
start_with?    0.200000   0.000000   0.200000 (  0.200284)
=~ (^)         0.280000   0.000000   0.280000 (  0.282534)
=~ (A)         0.300000   0.000000   0.300000 (  0.291691)
scan (^)       1.710000   0.010000   1.720000 (  1.728397)
scan (A)       1.700000   0.020000   1.720000 (  1.713138)