为什么=〜运算符有时仅具有副作用？

009

day

003

005

007

irb(main):001:0> r = /(?<day>\\d\\d):(?<mon>\\d\\d)/
=> /(?<day>\\d\\d):(?<mon>\\d\\d)/

irb(main):002:0> r =~ \"24:12\"
=> 0
irb(main):003:0> day
NameError: undefined local variable or method `day\' 

irb(main):004:0> \"24:12\" =~ r
=> 0
irb(main):005:0> day
NameError: undefined local variable or method `day\'


irb(main):006:0> \"24:12\" =~ /(?<day>\\d\\d):(?<mon>\\d\\d)/
=> 0
irb(main):007:0> day
NameError: undefined local variable or method `day\'


irb(main):008:0> /(?<day>\\d\\d):(?<mon>\\d\\d)/ =~ \"24:12\"
=> 0
irb(main):009:0> day
=> \"24\"

\"24:12\" =~ r

\"24:12\".=~(r)

/(?<day>\\d\\d):(?<mon>\\d\\d)/ =~ \"24:12\"

003

   re = /(?<lhs>\\w+)\\s*=\\s*(?<rhs>\\w+)/
   re =~ \"  x = y  \"
   p lhs    # undefined local variable
   p rhs    # undefined local variable

/(?<lhs>\\w+)\\s*=\\s*(?<rhs>\\w+)/