sub N_choose_R
{
	my( $l, $n, ) = @_;
	$n > $l and die "$n > $l !!!\n";
	$n == 1 and return( reverse map { [$_] } 0 .. $l-1 );
	$n == $l and return [ reverse( 0 .. $l-1 ) ];
	my @lists;
	for my $s ( reverse( $n-1 .. $l-1 ) )
	{
		push @lists, map { [ $s, @$_ ] } N_choose_R( $s, $n-1 );
	}
	@lists
}


my $motif = 'gccrccaugg';
my $anchor = 'aug';
my $max_mismatches = 3;

my( $pre, $post ) = split $anchor, $motif, 2;

my $anchor_ofs = length $pre;

my @patterns = map
{
	my $p = $motif;
	for my $i ( @$_ )
	{
		substr( $p, $i < $anchor_ofs ? $i : $i + length($anchor), 1 ) = '.';
	}
	$p =~ s/r/[ag]/g;
	$p
}
N_choose_R( length( $pre.$post ), $max_mismatches );

my $re = join '|', @patterns;