From: Charles Mills <cmills@...>
Date: 2005-02-13T14:09:58+09:00
Subject: Re: duplicated match question

shaobo.li@gmail.com wrote:
> Hi,
>
> I am new to Ruby,and confusing about following regular patterns:
>
> why pattern "(\w)\1" match duplicated letter "ll"  in "Hello"

(\w) matches a word character and stores it in the first 'register'.
\1 is the literal value of the first register.

So, /(\w)\1/ matches a word character followed by that same word
character.
irb(main):004:0> /(\w)\1/ =~ 'll'
=> 0
irb(main):005:0> /(\w)\1/ =~ 'aa'
=> 0
irb(main):006:0> /(\w)\1/ =~ 'bb'
=> 0
irb(main):007:0> /(\w)\1/ =~ '11'
=> 0
irb(main):008:0> /(\w)\1/ =~ '##'
=> nil
'#' is not a word character.

> and "(\w+)\1" match "ississ" in  "Mississippi"?

/(\w+)\1/ matches a sequence of word characters followed by that same
sequence of word characters.
In 'Mississippi' the (\w+) matches 'iss' and the \1 which resolves to
'iss' matches 'iss' a second time, giving a complete match of
/(\w+)\1/.

Also note:
irb(main):009:0> /(\w+)\1/ =~ 'ssissi'
=> 0
irb(main):010:0> $1
=> "ssi"
irb(main):011:0> $&
=> "ssissi"

Hope that helps,
Charlie