forgive me if i don't understand... but you want to search through a string of DNA/RNA checking whether the ratio of nucleotides fits a specific ratio? i don't think a regular expression would be the best way to do this. it seems like you'd have to count up the number of each nucleotide in the string and compare the total numbers and see whether they're "close enough" to the ratio (which is a matter of statistical analysis which i won't talk about here). is that what you're asking for?

jeff