Dear Wise Monks,

SHORT VERSION

How would you write a regex for a conditional statement that searches for a string of the same character (N) that stretches across multiple lines and then replaces this multi -line block of Ns with something else?

LONG VERSION

I am trying to write a program that reads an input file formatted as follows:

>scaffold00001
TGACCAGCGTTACATGTCAACGTTTTTTTAAACATTGAATATTTATTCATCTGTTCTCGA
CGTCGCACATTGTGCAAGCCATCAACTTGATCTAATTAAGTATATCCGCTAAAGATCAGC
GCCTTTTGTTTTGACAACAACAAGTATCATACCAGCACTTTAACGATGTAGTCAATAAAA
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
TGTCTGCTTCCGTTGATACCTGCGCGAGCCGGTCGTCACGATTCGGTACCTTGCCGTCAA
TCAAAAAATGGCAATGATGATCCAACAATGGGACTTGGTCGACAAACTCAGATAAGTCGT
CCAAAATACTTCCTCCTTCACACGTTTGCTGGTGCAATGACCAGCCCACGTTTTCCTAGA
>scaffold00002
ATTCATGTTGCTCGTCGTCACCTGACCGAGCAGGGCCAGAACCGGGACATGATCCATTTT
GGCATCATACAAACCATTTAATAGATGAATGGCGCCAGGGCCGCCAATGGAGAGACAAAC
ACCCAAACCGCCTGTTAACTTAGCAGTTGCCGCTGCGGACAAAGCGGCGACTTCCTCATG
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN
CAACTGCTCTTAGTGTGATCTCATTTATCAAAGAGATCAGACTTTGTGAATTCAGATTTT
TTCAAGCCATACAACAACTAACCCAATTTAAGAAATACCCACTTATTGCGATTTGCTTTT
CTATTAGTTAGCATTTTAAATTGTGAAACGTGCCACTTATAAACAAATTTCCGTCTTCTT
[download]

The script should then replace the lines that do not contain a G, A , T or C with a string that exits only on one line. The code I have so far is as follows:

#!/util/bin/perl
use strict;
use warnings;
use File::Copy;
#################


#-----Declaration of Arguements-----#

my $fasta=$ARGV[0] || die ("Please provide a 454Scaffolds.fna fasta fi
+le to parse.\n"); # Argument accepts scaffold fasta file.
my $tempfasta="temp.fasta";
my $workingfasta="working.fasta";
my $scaffold;                                                         
+                  # Initializing $scaffold globally.
my $enumerator;                                                       
+                  # Initializing $enumerator globally.

#----Main Program-----#

copy($fasta, $tempfasta) or die "Could not copy $fasta to $workingfast
+a.";



open (FILE,"<temp.fasta") || die ("Could not open fasta file!\n"); # O
+pen & read fasta file
open (WORKFILE,">>working.fasta");
while (my $fastaline=<FILE>)
  {
    if($fastaline=~ m/^>/)                                            
+ # IF first character in line is > assign line to $fastaline then...
    {
    my $scaffold_header=$fastaline;                                   
+  # set $scaffold_header to $fastaline then... 
    $scaffold_header=~ /(>)(\S*)(\s*)(\S*)/;                          
+  # divide $ scaffold header components then...
    $scaffold= $2;                                                    
+  # assign component $2 to $scaffold then...
    print WORKFILE ">" . "$scaffold" . ".1\n";                        
+  # print $scaffold to screen and start new line.
    $enumerator=1;
}  elsif ($fastaline!~ m/G/ and $fastaline!~ m/A/ and $fastaline!~ m/T
+/ and $fastaline!~ m/C/) 
  {
  $enumerator++; 
  print WORKFILE (">" . "$scaffold" .  ".$enumerator\n");  
} else {
  print WORKFILE $fastaline;
}
};
close (FILE);
close (WORKFILE);
[download]

So, the output I get with this code as is, is close to what I want but not exactly so. This is what the output looks like now:

>scaffold00001.1
TGACCAGCGTTACATGTCAACGTTTTTTTAAACATTGAATATTTATTCATCTGTTCTCGA
CGTCGCACATTGTGCAAGCCATCAACTTGATCTAATTAAGTATATCCGCTAAAGATCAGC
GCCTTTTGTTTTGACAACAACAAGTATCATACCAGCACTTTAACGATGTAGTCAATAAAA
>scaffold00001.2
>scaffold00001.3
>scaffold00001.4
TGTCTGCTTCCGTTGATACCTGCGCGAGCCGGTCGTCACGATTCGGTACCTTGCCGTCAA
TCAAAAAATGGCAATGATGATCCAACAATGGGACTTGGTCGACAAACTCAGATAAGTCGT
CCAAAATACTTCCTCCTTCACACGTTTGCTGGTGCAATGACCAGCCCACGTTTTCCTAGA
>scaffold00002.1
ATTCATGTTGCTCGTCGTCACCTGACCGAGCAGGGCCAGAACCGGGACATGATCCATTTT
GGCATCATACAAACCATTTAATAGATGAATGGCGCCAGGGCCGCCAATGGAGAGACAAAC
ACCCAAACCGCCTGTTAACTTAGCAGTTGCCGCTGCGGACAAAGCGGCGACTTCCTCATG
>scaffold00002.2
>scaffold00002.3
CAACTGCTCTTAGTGTGATCTCATTTATCAAAGAGATCAGACTTTGTGAATTCAGATTTT
TTCAAGCCATACAACAACTAACCCAATTTAAGAAATACCCACTTATTGCGATTTGCTTTT
CTATTAGTTAGCATTTTAAATTGTGAAACGTGCCACTTATAAACAAATTTCCGTCTTCTT
[download]

And this is how I WANT the output to look like:

>scaffold00001.1
TGACCAGCGTTACATGTCAACGTTTTTTTAAACATTGAATATTTATTCATCTGTTCTCGA
CGTCGCACATTGTGCAAGCCATCAACTTGATCTAATTAAGTATATCCGCTAAAGATCAGC
GCCTTTTGTTTTGACAACAACAAGTATCATACCAGCACTTTAACGATGTAGTCAATAAAA
>scaffold00001.2
TGTCTGCTTCCGTTGATACCTGCGCGAGCCGGTCGTCACGATTCGGTACCTTGCCGTCAA
TCAAAAAATGGCAATGATGATCCAACAATGGGACTTGGTCGACAAACTCAGATAAGTCGT
CCAAAATACTTCCTCCTTCACACGTTTGCTGGTGCAATGACCAGCCCACGTTTTCCTAGA
>scaffold00002.1
ATTCATGTTGCTCGTCGTCACCTGACCGAGCAGGGCCAGAACCGGGACATGATCCATTTT
GGCATCATACAAACCATTTAATAGATGAATGGCGCCAGGGCCGCCAATGGAGAGACAAAC
ACCCAAACCGCCTGTTAACTTAGCAGTTGCCGCTGCGGACAAAGCGGCGACTTCCTCATG
>scaffold00002.2
CAACTGCTCTTAGTGTGATCTCATTTATCAAAGAGATCAGACTTTGTGAATTCAGATTTT
TTCAAGCCATACAACAACTAACCCAATTTAAGAAATACCCACTTATTGCGATTTGCTTTT
CTATTAGTTAGCATTTTAAATTGTGAAACGTGCCACTTATAAACAAATTTCCGTCTTCTT
[download]

The problem is that the script is inserting the string I want to replace the Ns with for every line it sees them in, but I want it treat each consecutive line that has the string of N as a single unit. My guess is that I need to replace the:

 elsif ($fastaline!~ m/G/ and $fastaline!~ m/A/ and $fastaline!~ m/T/ 
+and $fastaline!~ m/C/)
[download]

With some kind of more complex regular expression. Being that I am new to regex, this is where I am stuck. Any help would be greatly appreciated!

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