Hi ruby experts!

Is this intended behaviour?

irb(main):001:0> s1='a=1'
=> "a=1"
irb(main):002:0> s2='b=1'
=> "b=1"
irb(main):003:0> s1 =~ /a|b=(.)/
=> 0                              <------ expression matches
irb(main):004:0> $1
=> nil                            <------ but where is argument?
irb(main):005:0> s2 =~ /a|b=(.)/
=> 0                              <------ expression matches
irb(main):006:0> $1
=> "1"                            <------ this has been expected
irb(main):007:0> s1 =~ /(a|b)=(.)/
=> 0                              <------ expression matches
irb(main):012:0> $2
=> "1"                            <------ this has been expected

Tested on ruby 1.8.2 (2004-12-22) [i686-linux]

Thanks for your help in advance
Martin.