However it would be so much easier to do it with Text::Balanced or Parse::RecDescent that its unlikely anyone would try. Here is a solution using Text::Balanced

use Text::Balanced 'extract_tagged';
use Data::Dumper;

sub get_ifs {
    my $str=shift;
    my $list=shift || [];

    my ($extracted,
        $remainder,
        $prefix,
        $open,
        $inside,
        $close)=extract_tagged($str,"IF","ENDIF","(?s).*?(?=IF)");
    if ($extracted) {
        push @$list,$extracted;
        get_ifs($inside,$list);
        get_ifs($remainder,$list) if $remainder;
    }
    return $list;
}

print Dumper(get_ifs(<<ENDOFIFS));
IF(A) anytext
    IF(B)
        IF(C) anytext ENDIF
        IF(D) anytext ENDIF
    ENDIF
ENDIF
ENDOFIFS
[download]

What my first sentence meant to say was:

A formal regular expression can only match an arbitrarily nested structure. Thus if we wanted to match to depth of say 6 we could do it, but if we want to match to any depth a formal regex wont work. And as I said above perls regexes arent formal regexes. They are decidedly irregular as they have support for backrefs.

Yves / DeMerphq
--
When to use Prototypes?

Thanks!

$_ = "
IF(A) anytext
IF(B) anytext ENDIF
IF(C) anytext ENDIF
and on ....
ENDIF
";
@( = ('(','');
@) = (')','');
($re=$_)=~s/((\bIF\b)|(\bENDIF\b)|.)/$([!$2]\Q$1\E$)[!$3]/gs;
print join"\n\n",eval{/$re/},"";
warn $@ if $@=~/unmatched/;
[download]