Function prototype to check escape sequence.

You can find more information in comments below.

/*
    Check escape symbols sequence function prototype 
    @param s String to look at 
    @param min Minimum allowed position to look at 
    @param max Maximum allowed position to look at 
    @param index Symbol index where to look at escape sequence 
    @param get_char Retreiving character function 
    @return New index(just after escape sequence) or index if there is no escape sequence at specified position 
    */
    using FEscape = int(*)(const String &s, const int max, const int index, FGetChar get_char);