在正则表达式中查找正则表达式（Python 3.5）

Question

I'm writing a script that returns whether or not a DNA sequence (with ambiguous letters) contains a certain subsequence (also with ambiguous letters).

I've tried creating a regular expression for both the sequence and the subsequence, but I haven't found a way to find if the latter occurs in the former. I need something like re.search(substring,sequence) but where sequence is another regular expression instead of a string.

Does such a thing exist?

Edit: what I've tried:

import re

def subseqof(substr,sequence):
    substr_exp = ''
    sequence_exp = ''
    for letter in substr:
        for code,expr in zip(['A','C','G','T','R','Y','S','W','K','M','B','D','H','V','N','X'],
                           ['A','C','G','T','[AG]','[CT]','[GC]','[AT]','[GT]','[AC]','[CGT]','[AGT]','[ACT]','[ACG]','[ACGT]','[ACGT]']):
            if letter == code:
                substr_exp += expr
    for letter in sequence:
        for code,expr in zip(['A','C','G','T','R','Y','S','W','K','M','B','D','H','V','N','X'],
                           ['A','C','G','T','[AG]','[CT]','[GC]','[AT]','[GT]','[AC]','[CGT]','[AGT]','[ACT]','[ACG]','[ACGT]','[ACGT]']):
            if letter == code:
                sequence_exp += expr
    if re.search(substr_exp,sequence_exp) != None:
        return True
    else:
        return False

Answer 1

I need something like re.search(substring,sequence) but where sequence is another regular expression instead of a string.

If I understand correctly, you want eg subseqof(substr='RR', sequence='...AA...') as well as subseqof(substr='AA', sequence='...RR...') to return True (where in both cases R is replaced with [AG] ).

Does such a thing exist?

Such a symmetric or commutative regular expression function doesn't exist. But as only a small subset of regex functionality is used here, we can easily write a proper function:

iupac = dict(zip(['A','C','G','T', 'R', 'Y', 'S', 'W', 'K', 'M',  'B',  'D',  'H',  'V',   'N',   'X'],
                 ['A','C','G','T','AG','CT','GC','AT','GT','AC','CGT','AGT','ACT','ACG','ACGT','ACGT']))

def subseqof(substr, sequence):
    substr_exp   = list(map(iupac.get, substr))   # list of letters represented
    sequence_exp = list(map(iupac.get, sequence)) # list of letters represented
    for start in range(len(sequence_exp)-len(substr_exp)+1):    # sequence loop
        for subpos, sub in enumerate(substr_exp):           # sub-sequence loop
            if not sequence_exp[start+subpos]: break    # if letter not in dict
            if not re.search('['+sub+']', sequence_exp[start+subpos]): break
        else: return start                          # whole subsequence matched