perl regex referencing

ocs has asked for the wisdom of the Perl Monks concerning the following question:

Hello monks,

I got a small problem concerning references in perls regex flavor.
For example, I got a string like this

$lala = "Hello you therre."

And I want to match all double characters (a-z), i.e. ll and rr in the given string and contract them to one char. (Please do not take this seriously, its just an example.)

So I went like this:

$lala =~ s/([a-z])$1/$1/g

It does not work. So I played around:

$lala =~ s/([a-z])\1/$1/g

This works. But what I kept in mind was this: Warning on \1 vs $1

So I don't know why in the first version the $1 in the matching part (not the substitute part) does not work but with \1 in the second version? I thought \1 is obsolete and just a relic of a sed styled referencing.

This is nothing big, but ... did I miss something?

Thanks in advance,

ocs.

tennis players have fuzzy balls.

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Re: perl regex referencing by zshzn (Hermit) on Sep 18, 2007 at 08:26 UTC
perlre explains this as `The bracketing construct ( ... ) creates capture buffers. To refer to +the digit'th buffer use \<digit> within the match. Outside the match +use "$" instead of "\".` [download]	[reply] [d/l]
Re: perl regex referencing by Prof Vince (Friar) on Sep 18, 2007 at 08:22 UTC
It's not a problem to use `\1` and friends in the LHS of the substitution because it doesn't behave like a quoted string : the <backslash-digit> token has a well defined meaning there. Moreover, `$1` can already be a capture in a previous successful regexp, so it shouldn't be reset before the end of the matching part of the substitution.	[reply] [d/l] [select]
Re: perl regex referencing by bruceb3 (Pilgrim) on Sep 18, 2007 at 08:18 UTC
The back slash is used inside the match and the dollar sign is used outside of the match. In this case "inside the match" refers to (a-z)\1 because the text is being matched against this regex. The dollar 1 is not being matched against. It's part of the substitution.	[reply]
Re: perl regex referencing by ikegami (Patriarch) on Sep 18, 2007 at 14:38 UTC
Some alternatives: ≥ 5.10 `$lala =~ s/([a-z])\K\1//g;` [download] See the `perlre` for 5.10 (or the one for 5.9.5). < 5.10 `use Regexp::Keep; $lala =~ s/([a-z])\K\1//g;` [download] See Regexp::Keep. Update: Added links to documentation as privately requested.	[reply] [d/l] [select]
Re: perl regex referencing by eff_i_g (Curate) on Sep 18, 2007 at 14:16 UTC
`$var =~ tr/a-z//s;` [download] :)	[reply] [d/l]


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