3/sirna.pl

# First read the sequence from a file named 'mrna.fa'
$seq = '';
open(IN, 'mrna.fa') or die "Could not open file mrna.fa\n";
while (<IN>) {
    unless (/>/) {
        chomp;
        $seq .= $_;
    }
}
close IN;

# Now analyze the sequence read from file
# Step through each position  of the sequence
for (  $i = 0 ; $i < length($seq) -22 ; $i++ ) {
     $testseq = substr( $seq, $i, 23 );

    # check if first two positions are AA and
    # last are TT
    if ( $testseq =~ /^AA.*TT$/ ) {

         # test GC content

         # count the number of G's and C's
         $gc_content = ( $testseq =~ tr/GC// ) / 23;
        
         # is the GC content within the range 30-50?
         if ( ( $gc_content >= 0.3 ) && ( $gc_content <= 0.5 ) ) {

            # does the sequence contain stretches of As, Ts, Cs or Gs?
            unless ( ( $testseq =~ /A{4}/ )
                || ( $testseq =~ /T{4}/ )
                || ( $testseq =~ /G{4}/ )
                || ( $testseq =~ /C{4}/ )

            # avoid also regions of six positions with G or C
                ||  ( $testseq =~ /[GC]{6}/ ))
            {
                print "pos $i $testseq\n";
            }
        }
    }
}