From: ahoward <ahoward@...>
Date: 2003-04-30T14:13:10+09:00
Subject: Re: Ruby regexp backreferences

On Wed, 30 Apr 2003, Austin Ziegler wrote:

> I'm doing something that required RLE, and the code that I
> translated from Perl to do this included the following regexp:
>
>   /^(.*?)((.)\2{2,127})(.*?)$/ois
<snip>

austin - are you sure this works?  it's odd because

>   /^(.*?)((.)\2{2,127})(.*?)$/ois
           ^^^^^^^^^^^^^^
to me this seems to say, "the second match shall be composed of a single char
followed 2 to 127 of the second match."  in otherwords, it would seem to be
recursive.

for instance, this does not work in perl or ruby:

  ~ > cat foo
  'foo' =~ /^(.)((.)\2)/;
  print $1,"\n";
  print $2,"\n";
  print $3,"\n";
  > perl foo



  > ruby foo
  nil
  nil
  nil

but this does

  ~ > cat foo
  'foo' =~ /^(.)((.)\3)/;
  print $1,"\n";
  print $2,"\n";
  print $3,"\n";
  > perl foo
  f
  oo
  o
  > ruby foo
  f
  oo
  o

i guess what i am saying is that i don't see how the original match ever had
valid semantics and that, if it worked, it would seem to imply a broken perl.

in any case - ruby's behaviour seems correct.

> While what's happening makes sense, I'm wondering if it's correct -- how
> deep should backreferences be nested and considered part of the process?

i think the only meaningful way is for *each* '(' which is not escaped, or
followed by a '?:' to begin the '\n' and '$n' groups - without limiting the
depth.

-a

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