I have a problem I cannot understand. I have this string: gene_id "siRNA_Z27kG1_20543"transcript_id "siRNA_Z27kG1_20543_X_1";tss_id "TSS124620" And I want to change the gene_id. So, I have the following code: if ($line =~ /;transcript_id "([A-Za-z0-9:\-._]*)(_[oxOX][_.][0-9]*)";/) { $num = $2; $line =~ s/gene_id "([A-Za-z0-9:\-._]*)";/gene_id "$1$num";/g; print $new $line."\n"; } The aim of my code is to change siRNA_Z27kG1_20543 for siRNA_Z27kG1_20543_X_1. However, my code does not produce that output. Why? I can't understand that. My regex needs to be as it is because I match other strings (this time with success). Thanks.