查找字符索引的文件偏移量，忽略换行符

I have a text file of 3GB size (a FASTA file with DNA sequences). It contains about 50 million lines of differing length, though the most lines are 70 characters wide. I want to extract a string from this file, given two character indices. The difficult part is, that newlines shall not be counted as character. I have a text file of 3GB size (a FASTA file wi