Benchmark

$ ruby -v
ruby 2.1.1p76 (2014-02-24 revision 45161) [x86_64-linux]

$ ruby re_bench.rb 
Rehearsal --------------------------------------------------------
(?-mix:^[a-zA-Z]$)     0.780000   0.000000   0.780000 (  0.783924)
(?i-mx:^[a-z]$)        0.790000   0.000000   0.790000 (  0.789410)
(?-mix:\A[a-zA-Z]\z)   0.370000   0.000000   0.370000 (  0.373894)
(?i-mx:\A[a-z]\z)      0.370000   0.000000   0.370000 (  0.373349)
----------------------------------------------- total: 2.310000sec

                           user     system      total        real
(?-mix:^[a-zA-Z]$)     0.770000   0.000000   0.770000 (  0.773134)
(?i-mx:^[a-z]$)        0.790000   0.000000   0.790000 (  0.787266)
(?-mix:\A[a-zA-Z]\z)   0.380000   0.000000   0.380000 (  0.375982)
(?i-mx:\A[a-z]\z)      0.380000   0.000000   0.380000 (  0.374873)

re_bench.rb

require 'benchmark'

N       = 1_000_000
STRING  = ['a'..'z', 'A'..'Z'].map(&:to_a).sample(2).join.freeze

def test_match(re)
  STRING =~ re
end

CS_OLD  = /^[a-zA-Z]$/
CIS_OLD = /^[a-z]$/i

CS_NEW  = /\A[a-zA-Z]\z/
CIS_NEW = /\A[a-z]\z/i

Benchmark.bmbm(15) do |x|
  [CS_OLD, CIS_OLD, CS_NEW, CIS_NEW].each do |re|
    x.report(re.to_s) { N.times { test_match re } }
  end
end