I need to find the following pattern from a genomic sequence

B represents C, G, or T
N15 represents A, T, G or C and these four base combination should have the length of 15 bases.

To do the same, I tried grep command, but I could not specify B and N15 in the grep command line. Therefore, please help me to find the same.

For example, I have a nucleotide sequences as follows


Expected outcome,


Thanks in advance.

  • 1
    $\begingroup$ Could you please update your question to clarify that N15 represents A, T, G or C for exactly 15 bases? $\endgroup$ – gringer Nov 5 '19 at 23:00
  • [CGT] looks for one occurrence of one of the three.
  • [ATGC]{1,15} looks for "up to 15 letter long" combinations of the four bases.

Moreover, -P makes your regex call Perl-like and is required to make this regex work. Different regex flavors behave (slightly) differently.

  • $\begingroup$ Dear @haci, I have followed your command line for the test data (I had given in the example section of my question), which gives proper result. But when I tried the same with my genome file with the following command, echo $(<org15.fasta) | grep -oP 'TTCG[CGT][ATGC]{1,15}TTCG[CGT]' and I am getting output as follows, TTCGCCGCGGCGACGTTTTCGC TTCGCATTCGG TTCGCCACCTTGTTCGG TTCGGCTTTCGC TTCGTGGCCTTCGG TTCGGCTTCTTCGT TTCGTCAAGGACCCGATTTCGG TTCGCATCTTCGC $\endgroup$ – Kumar Nov 5 '19 at 11:48
  • $\begingroup$ The thing is that, the script mining TTCGB.....TTCGB, but fails to fulfil the length N15 criteria. kindly help me to fix this. Thank you. $\endgroup$ – Kumar Nov 5 '19 at 11:50
  • $\begingroup$ What is your "N15 criteria" though? [ATGC]{1,15} looks for up to 15 repetitions of the four bases as you specified in your question. And as far as I can see in the output you get, this is what you are getting, 2 TTCGB stretches are connected by up to 15 bases. $\endgroup$ – haci Nov 5 '19 at 11:55
  • 2
    $\begingroup$ N15 usually means exactly 15 of any nucleotides. So use [ACGTN]{15} . Also, -i flag for grep may need to be added if some of the sequence is lower case, for example repeat masked. $\endgroup$ – Timur Shtatland Nov 5 '19 at 13:01
  • 2
    $\begingroup$ OK, then just change [ATGC]{1,15} to [ATGC]{15} as Timur Shtatland suggested. The reason for [ATGC]{1,15} was your phrase in the question: "N represents A, T, G or C up to 15 bases". $\endgroup$ – haci Nov 5 '19 at 13:50

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