few doubts on regular expressions

 Hi,

     I was studing the manual of regular expressions. I have 3 questions regarding it:

1) How rexExecute is resulting 't' in first case & 'nil' in second ? 

rexCompile("\\([a-z]+\\)\\.\\1")        => t
rexExecute("abc.bc")                    => t
rexExecute("abc.ab")                    => nil

2)How the rexSubstitute is able to result only the numaric '123' ?

Is the register \0 stores the entire pattern "abc123" & \1 stores only "123" ?  How exactly does register storing happen ?

rexCompile( "[a-z]+\\([0-9]+\\)" ) => t
rexExecute( "abc123" )             => t
rexSubstitute( "*\\0*" )            => "*abc123*"
rexSubstitute( "The matched number is: \\1" )
                                    => "The matched number is: 123"
 

 3)Can anyone explain the following synopsis ?

[synopis] rs : [meaning] A composite regular expression rs matches the longest match of r followed by a match for s.