Finding the right $<*digit*> capture variable

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

If I write a regex which includes multiple capturing groups (qr/(\D+)(\d+)/ and (?{ }) eval blocks is there a way to find out which numbered variable was just assigned to? So if I wrote qr/(\D+)(\d+)(\D+)(?{ foo( ?? ) })/ is there a way to find out that I should be reading from $3 and not a different variable?

I tried writing some diagnostic code to compare the addresses of $+ and the numbered variables but that didn't do anything useful. It indicated that each of $+, $1, $2 and $3 are all separate variables and $+ isn't temporarily aliased to $3 which would at least let me compare references like \ $+ == \ $1. So what works?

$text = "abc123def";
$text_match = qr/(\D+)(\d+)(?{ dumpr() })(\D+)/;
$text =~ $text_match;

use Devel::Peek;
sub dumpr {
    Dump($+), Dump($1), Dump($2), Dump($3);
    local $\ = "\n";
    print $+;
    print $1;
    print $2;
    print $3;
}
[download]

Comment on Finding the right $<digit> capture variable Select or Download Code

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Re: Finding the right $<digit> capture variable by Enlil (Parson) on Apr 15, 2003 at 22:34 UTC
I don't know where you are going with this but the `$^N` variable, but it might be what you are looking for (granted I am aware that it does not get you the number). According to perlvar: This is primarily used inside `(?{...})` blocks for examining text recently matched. For example, to effectively capture text to a variable (in addition to `$1`, `$2`, etc.), replace `(...)` with `(?:(...)(?{ $var = $^N }))` [download] By setting and then using $var in this way relieves you from having to worry about exactly which numbered set of parentheses they are. -enlil	[reply] [d/l] [select]
Re: Re: Finding the right $<digit> capture variable by diotalevi (Canon) on Apr 16, 2003 at 05:01 UTC
Not only is `$^N` specific to >=5.8.0 it doesn't provide any information regarding which numbered variable it refers to. Thank you though.	[reply] [d/l]
Re: Finding the right $<digit> capture variable by Enlil (Parson) on Apr 16, 2003 at 09:09 UTC
Let me try again. How about `$#-` . Again from perlvar: One can use $#- to find the last matched subgroup in the last successful match. Contrast with $#+, the number of subgroups in the regular expression. Some code that might or not result in what you want (as I am still not sure where you are going with this.) Read more... (682 Bytes) -enlil	[reply] [d/l] [select]
Re: Finding the right $<digit> capture variable by tekkie (Beadle) on Apr 16, 2003 at 14:07 UTC
I'm not sure where you're going with this either, `$#-` probably does what you're looking for. You could also write a subroutine to check which `$<DIGIT>` values are defined, and return the last one found: `sub last_match { # Accepts no args # Returns the digit of the last parenthesis match or undef if ther +e isn't one my $match_num = 1; if(defined($$match_num)) { while(defined($$match_num)) { $match_num++; } $match_num--; } else { undef $match_num; } return $match_num; }` [download]	[reply] [d/l] [select]
Re: Re: Finding the right $<digit> capture variable by hv (Prior) on Apr 17, 2003 at 13:26 UTC
Beware that this approach cannot discover the last defined match variable, since there may be gaps in the list: for example after `"b" =~ /(a)?(b)/`, `$1` will not be defined even though `$2` is. Since there is no useful absolute limit on the highest numbered match variable you might need to check for, I wondered whether walking the symbol table might give a clue, but no such luck - the symbol table entries for `*1` etc are created only if they are explicitly referenced in the code: `"abcde" =~ /(.)(.)(.)(.)(.)/; my $var = $4; print join ', ', grep !/\D/, keys %::;` [download] prints "`0, 4`". Accordingly, I think it is not possible in pure perl to discover the highest numbered defined match variable using any perl before v5.6.0 (when `@+` and friends were first introduced). Hugo	[reply] [d/l] [select]
Re: Re: Re: Finding the right $<digit> capture variable by diotalevi (Canon) on Apr 17, 2003 at 13:48 UTC
I was thinking of taking the sideways route - instrument the ')' part of a '(...)' group so that the contents of @+ is copied elsewhere for safekeeping. Any time that I'm interested in knowing which newly closed group was just passed I'd look for the index of the newly defined value in @+. So after `(a)?` `$+[1]` and `$+[2]` are undefined. After `(b)` `$+[1]` is still undefined but `$+[2]` is defined (and equal to 2). So... if I can know which @+ entry was just created is that sufficient or will that break as well?	[reply] [d/l] [select]
Re^4: Finding the right $<digit> capture variable by hv (Prior) on Apr 17, 2003 at 14:29 UTC


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