Cool. I know nothing about "automata", even how it is translated to my native language.
But seems you answered already of what should be known to come to such solution. Thank you :).

Because the chars in text should be assigned to, otherwise a copy would be assigned to.

In fact it is creating reference to char instead of deep copy of char. But considering char is small (1 byte guaranteed) it does not have any benefit here. And how guy bellow mentioned because I want to modify the string.

Thank you very much!

The function returns a function not void
The solution is returning a lambda expression which is the shortcut version for writing a function

Thank you very much!

It's similiar to foreach (int i in idxs)

for(int i : idxs)

Could you please tell me what does it mean?
I don't understand "i : idxs"...