Validating credit card

[frequency]

done

def checker(s):
    list_of_numbers=[]
    string_to_use=""
    if("-" in s):
        list_of_numbers=s.split("-")
        for character in list_of_numbers:
            string_to_use+=character
        string_return=string_to_use
        string_to_use=""
        return string_return
    if(" " in s):
        list_of_numbers=s.split(" ")
        for character in list_of_numbers:
            string_to_use+=character
        string_return=string_to_use
        string_to_use=""
        return string_return
    if("," in s):
        list_of_numbers=s.split(",")
        for character in list_of_numbers:
            string_to_use+=character
        string_return=string_to_use
        string_to_use=""
        return string_return
    if("*" in s):
        list_of_numbers=s.split("*")
        for character in list_of_numbers:
            string_to_use+=character
        string_return=string_to_use
        string_to_use=""
        return string_return

    if("." in s):
        list_of_numbers=s.split(".")
        for character in list_of_numbers:
            string_to_use+=character
        string_return=string_to_use
        string_to_use=""
        return string_return
    else:
        return s

credit_card=[]
credit_card_sum=[]
credit_card_sum_digit=[]
credit_digits_sum=0

user1_input=input()
user_input=checker(user1_input)
check=False
while user1_input!="end" and user1_input!="END":
    user_input=checker(user1_input)
    if(len(user1_input)>16):
        if(len(user1_input)!=19):
            print("False")
            user1_input=input()
            continue
        elif(user1_input[4].isdigit() or user1_input[9].isdigit() or user1_input[14].isdigit()):
                print("False")
                user1_input=input()
                continue
    if( not user_input.isdigit()):
        print("False")
    elif(user_input.isdigit() and len(user_input)!=16):
        print("False")
    else:
        credit_card_int=[int(x) for x in user_input]
        if credit_card_int[0]<4 or credit_card_int[0]>7:
            print ("False")
        else:
            for i in range(0,len(credit_card_int),2):
                credit_card_int[i] = credit_card_int[i]*2
                if credit_card_int[i]>=10:credit_card_int[i]= (int(str(credit_card_int[i])[0])+int(str(credit_card_int[i])[1]))
            if (sum(credit_card_int) % 10 ==0):
                print ("True")
            else:
                print ("False")
    user1_input=input()