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

Hi, using @- and @+ I can get the _offset_ information of any capture porvided I know its ordinal number in the capture list, e.g. $-[2] for (un-named) backref $2. If I work with named backrefs, e.g. (?<myref>), I can easily get its _content_ via $+{myref}. Now, I am seeking to access the _offset_ values of a named backref given its name (here: 'myref') only ? An alternative route would be knowing the ordinal capture number corresponding to a named backref (such that I could access its offset information via the @-/@+ lists). Any idea how to do this ?

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Re: offset of named capture buffer
by Athanasius (Archbishop) on Nov 06, 2012 at 13:51 UTC

    See also the 2008 thread Finding match offsets of named captures in 5.10 regex, in which it is suggested that a code block can be used to capture the offset with a call to pos.

    Update: An example:

     2:56 >perl -Mstrict -wE "my $s = q[My 25 cents.]; my ($p, $q); $s =~ 
+/(?<myref>(?{$p = pos})\d+(?{$q = pos}))/; say $+{myref}; say $p, q[ 
+--> ], $q;"
25
3 --> 5

 2:56 >

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    Athanasius <°(((>< contra mundum

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Re: offset of named capture buffer
by Anonymous Monk on Nov 06, 2012 at 13:43 UTC
[reply]
      esp see http://perl5.git.perl.org/perl.git/blob?f=regcomp.c#l6532
      6524     if (rx && RXp_PAREN_NAMES(rx)) {
6525         HE *he_str = hv_fetch_ent( RXp_PAREN_NAMES(rx), namesv, 0
+, 0 );
6526         if (he_str) {
6527             IV i;
6528             SV* sv_dat=HeVAL(he_str);
6529             I32 *nums=(I32*)SvPVX(sv_dat);
6530             for ( i=0; i<SvIVX(sv_dat); i++ ) {
6531                 if ((I32)(rx->nparens) >= nums[i]
6532                     && rx->offs[nums[i]].start != -1
6533                     && rx->offs[nums[i]].end != -1)
6534                 {
6535                     ret = newSVpvs("");
6536                     CALLREG_NUMBUF_FETCH(r,nums[i],ret);
6537                     if (!retarray)
6538                         return ret;

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Re: offset of named capture buffer
by AnomalousMonk (Archbishop) on Jan 30, 2013 at 13:11 UTC

    Per perlre:

    "(?<NAME>pattern)"
        A named capture group. Identical in every respect to normal
        capturing parentheses "()" but for the additional fact ...

    Don't know if this helps much, but if you already know the ordinal number of the capture group associated with a named capture, then  @- @+ work exactly as advertised. (But it sounds as if you want to go from the named group to its capture group ordinal.) 

    >perl -wMstrict -le
"my $s = 'xxx fooooo yyy';
 ;;
 $s =~ m{ (x+) .*? (?<FOO>fo+) .*? (y+) }xms;
 ;;
 print qq{'$1'  '$+{FOO}'  '$3'};
 print qq{'$1'  '$2'  '$3'};
 print qq{/fo+/ at $-[2] to $+[2]};
"
'xxx'  'fooooo'  'yyy'
'xxx'  'fooooo'  'yyy'
/fo+/ at 4 to 10

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