#! perl -slw
use strict;
use re 'eval';

sub Re_Stream {
	my( $re_user, $extend ) = @_;
	die "Usage: Re_Stream( regex, coderef )"
		unless defined $re_user and ref $extend eq 'CODE';

	return qr[ (?: \Z (?(?{ $extend->() })|(?!) }) ) | $re_user ]x;
}

my $buf = 'abcdefghijklmnopqrstuvwxyz';
my $c = 'A';
sub extend{
	$buf .= ($c++) x 100;
	return length $c < 2
}

my $re_stream = Re_Stream( qr[(..)(...)], \&extend );
print $re_stream;

my $i = 0;
print "${ \++$i }: $1|$2" while $buf =~ m[$re_stream]g;