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

Can one (or more to the point, I) use a variable as a quantifier value in a regex ? As illustrated in below... 

#!/usr/bin/perl -w

use constant minUC => 3;
my $min = 3;
my $test = "AAAAAA";

    my @chars = (split //, $test);
    my $Rep = 0;
    foreach my $c ( @chars)
    {
        $Rep++  if  ($test =~ /$c{3,}?/) ;       # this works
#    $Rep++ if ($test =~ /$c{$min,}?/) ;   #     no work; 
#    $Rep++ if ($test =~ /$c{minUC,}?/) ; #nor does using a named cons
+tant   
    }

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Replies are listed 'Best First'.
Re: variable quantifiers?
by AnomalousMonk (Archbishop) on Jan 06, 2014 at 21:57 UTC 
    >perl -wMstrict -le
"my $c   = 'A';
 my $min = 3;
 ;;
 my $s = 'uuAvvAAwwAAAxxAAAAyy';
 ;;
 printf qq{'$1' } while $s =~ m{ ([[:lower:]] (?:$c){$min,} [[:lower:]
+]) }xmsg;
"
'wAAAx' 'xAAAAy'

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    Update: The quantified sub-pattern in  (?:$c){$min,} needs to be enclosed in a group (non-capturing or capturing) because otherwise  $c{$min,} looks to Perl too much like the interpolation of a scalar hash element from the  %c hash.

    Further Update: Interesting. I was unaware of the  \ (backslash) disambiguation pointed out below.

    Further Further Update: Or maybe not so interesting, since the regex doesn't actually seem to match anything: 

    >perl -wMstrict -le
"my $c   = 'A';
 my $min = 3;
 $_ = 'AAAAAA';
 print 'match' if /($c\{$min,})/;
 print qq{'$1'};
"
Use of uninitialized value $1 in concatenation (.) or string at -e lin
+e 1.
''

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    Yet Another Update: It is also possible to use a constant, but it's kinda messy. A Readonly scalar might do a better job for you. 

    >perl -wMstrict -le
"use constant MIN => 3;
 my $c = 'A';
 ;;
 my $s = 'uuAvvAAwwAAAxxAAAAyy';
 ;;
 printf qq{'$1' }
   while $s =~ m{ ([[:lower:]] (?:$c){${ \MIN },} [[:lower:]]) }xmsg;
"
'wAAAx' 'xAAAAy'

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[reply]
[d/l]
[select]
      $ perl -e " use re 'debug'; $c='a'; $min=3; m/$c\{$min,}/; "
Compiling REx "a\{3,}"
Final program:
   1: EXACT <a{3,}> (4)
   4: END (0)
anchored "a{3,}" at 0 (checking anchored isall) minlen 5
Freeing REx: "a\{3,}"

      [download]
[reply]
[d/l]
Re: variable quantifiers? (TIMTOWTDI)
by LanX (Saint) on Jan 07, 2014 at 01:23 UTC
    space separation under /x flag seems to do the job 
      DB<113> $_="AAAA";  $c="A"; $min=3; /$c {$min}/x; $&
 => "AAA"

    [download]
    UPDATE
    TIMTOWTDI 
      DB<108> $c="A";$min=3
 => 3

  DB<109> "AAAAA" =~ m/${c}{$min}/;  $&
 => "AAA"

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[d/l]
[select]
Re: variable quantifiers?
by lidden (Curate) on Jan 07, 2014 at 06:32 UTC
    There is always eval STRING if nothing else works.
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[d/l]
      The problem is the ambiguity of '$c{$min}' like AnomalousMonk already explained.

      eval also only operates on strings. So it won't allow to solve this variable interpretation problem differently, it'll only add more complexity.

      Cheers Rolf

      ( addicted to the Perl Programming Language)

[reply]
      Not in this case... Try and see!
[reply]

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