The Java program implements a finite state automaton (FSA) to validate whether a given string is a valid identifier based on specific rules. An identifier can start with a letter and can contain letters, digits, and underscores. It cannot start with a digit or underscore and cannot contain any other characters. The program checks each character in the input string against predefined rules to determine its validity as an identifier.
The program uses a state-based approach to validate identifiers:
Q0: Initial state Q1: State after reading the first character (letter) Q2: State for continuing valid characters (letters or digits) Q3: State for digits after the first character (letter) Q4: State for underscores after a letter or digit Q5: Trap state (invalid state)
| Current State | Letter (0) | Digit (1) | Underscore (2) | Other (3) |
|---|---|---|---|---|
| Q0 | Q1 | Q5 | Q5 | Q5 |
| Q1 | Q1 | Q2 | Q4 | Q5 |
| Q2 | Q2 | Q2 | Q4 | Q5 |
| Q3 | Q2 | Q2 | Q4 | Q5 |
| Q4 | Q4 | Q4 | Q4 | Q5 |
| Q5 | Q5 | Q5 | Q5 | Q5 |