gistfile1.txt

def temp(line,flag,pos,lvl=0):
    if line[pos].isdigit():
        flag[pos]=lvl+1
        if pos+1<len(line):
            return temp(line,flag,pos+1,lvl+1)
        else:
            return pos+1,lvl
    else:
        flag[pos]=0
        lvl=0
        return pos+1,lvl

def extract_digits(line):
    """
    extract longest digit sub-str from line
    :param line:
    :return:
    """
    flag=[0 for x in range(len(line))]
    print flag
    pos=0
    while pos<len(flag):
        pos,lvl=temp(line,flag,pos)
        print flag
    length=max(flag)
    if length>0:
        pe=flag.index(length)
        return line[pe-length+1:pe+1]
    else:
        return ''

if __name__=='__main__':
    print extract_digits('tt2012g2t11111g')