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Can someone please help me understand how regex works in line 34 which finds out the max consecutive STRs (sub-strings) in a string s

Specific line in code

str.update({k: max([len(x) // len(f'{k}') for x in re.findall(rf'((?:{k})+)', s)])})

I found the above syntax at Count max consecutive RE groups in a string

My complete code

from sys import argv, exit
import csv
import collections
import re


def main():
    if len(argv) != 3:
        print("Usage: python dna.py data.csv sequence.txt")
        exit(1)
    # store dna string to be matched in string 's'
    sequence = open(argv[2], 'r')
    s = sequence.read()
    sequence.close()

    # read the dna sequence of people from govt database
    columns = {}
    csvfile = open(argv[1], newline='')
    reader = csv.DictReader(csvfile, delimiter=',')
    # store column names in a list
    columns = reader.fieldnames
    database = list(reader)

    # create a dna dictionary key:value = column name : count of max consecutive column names in s (default value is 0)
    str = {}
    for i in range(1, len(columns), 1):
        str.update({columns[i]: "0"})

    # find out the max consecutive column name in s and then update the value in dictionary key:value = column name : count of max consecutive column names in s
    for k in str:
        if(re.findall(rf'((?:{k})+)', s) == []):
            str.update({k: '0'})
        else:
            str.update({k: max([len(x) // len(f'{k}') for x in re.findall(rf'((?:{k})+)', s)])})

    # match dictionary key:value = column name : count of max consecutive column names in s WITH column names' values in database
    # if match is made then print the name of the person from database whose dna has been matched
    for i in range(len(database)):
        match = True
        for j in range(1, len(columns), 1):
            if(int(database[i][f'{columns[j]}']) != int(str[f'{columns[j]}'])):
                match = False
        if(match):
            name = database[i]['name']
            break
        else:
            name = 'No match'
    print(name)


main()
Kartik Datwani
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  • 1
  • 'HELLOHELLOHELLOPPLF` Here the expression would return 3 for finding consecutive `HELLO`s The three would be `len(x) // len(str(k))` where x is the string of 3 consecutive `HELLO`s and k is the word 'HELLO`. (15//5 == 3) – Chris Charley Aug 09 '20 at 18:03
  • `for x in re.findall(rf'((?:{k})+)', s)` k is expanded by the `f` format specifier (and the 'r' specifier is really unnecessary as the are no backslashes here). Also, the capturing parenthesis are not necessary. Could be written as `for x in re.findall(f'(?:{k})+', s)` – Chris Charley Aug 09 '20 at 18:38
  • Thanks Chris. I actually understand this part. What I don't understand is the use of ?, :, +. What do these do and how does this combination ensure that only consecutive str counts will be calculated? – Kartik Datwani Aug 12 '20 at 09:01
  • **`(?: ...)+`** is a non capturing group repeating 1 or more (`+`) times. If `k` is 'ABC', this code will try to find all instances where 'ABC' repeats consecutively. – Chris Charley Aug 12 '20 at 16:39

0 Answers0