C++ :: Finding String Of Number Present Inside Brackets After A Keyword In File

I am reading a file whick looks like following:

Code:
10 = NAND(1, 3)
11 = NAND(3, 6, 5)
15 = NAND(9, 6, 2, 8)

Problem: I have to find the word "NAND" and then find the numbers inside the brackets because they are the inputs to that NAND gate. I have written a code below but that code can detect the fixed number of inputs. I need a code which can detect any number of inputs (whether 2 inputs or more than two). But i don't understand how do i do that?

My code:

Code:
string input_str ("INPUT"), output_str ("OUTPUT"), nand_str("NAND");
while (getline( input_file, line ))
{
std::size_t guard_found = line.find(guard_str);

[Code].....