Is it possible to use a back reference to specify the number of replications in a regular expression? foo= 'ADCKAL+2AG.+2AG.+2AG.+2AGGG+.+G+3AGGa4.' The substrings that start with '+[0-9]' followed by '[A-z]{n}.' need to be replaced with simply '+' where the variable n is the digit from earlier in the substring. Can that n be back referenced? For example (doesn't work) '+([0-9])[A-z]{/1}.' is the pattern I want replaced with "+" (that last dot can be any character and represents a quality score) so that foo should come out to ADCKAL++++G.G+. foo = 'ADCKAL+2AG.+2AG.+2AG.+2AGGG^+.+G+3AGGa4.' indelpatt = re.compile('\+([0-9])') while indelpatt.search(foo): indelsize=int(indelpatt.search(foo).group(1)) new_regex = '\+%s[ACGTNacgtn]{%s}.' % (indelsize,indelsize) newpatt=re.compile(new_regex) foo = newpatt.sub("+", foo) I'm probably missing an easier way to parse the string.