Example usage of a State Transition Table. Checks if the input string has the form of a valid unsigned integer.

main.py

"""
State transition table for scanning an unsigned int.

Source: p. 34, Engineering a Compiler, 3ed.
"""

S0 = 0
S1 = 1
S2 = 2
SE = 3

table = [
    [S1, S2, S2, S2, S2, S2, S2, S2, S2, S2],
    [SE, SE, SE, SE, SE, SE, SE, SE, SE, SE],
    [S2, S2, S2, S2, S2, S2, S2, S2, S2, S2],
    [SE, SE, SE, SE, SE, SE, SE, SE, SE, SE],
]

def next_state(state, char):
    index = ord(char) - ord("0")
    if index >= 0 and index <= 9:
        return table[state][index]
    else:
        return SE

def print_error(string, state, cursor):
    column = cursor - 1
    print(f"Error: Unexpected symbol at column {column}.")
    print(f"{string}\n{'^'.rjust(column)}")

def is_uint(string):
    buffer  = string + "\0"
    cursor = 0
    
    state = S0
    char  = buffer[cursor]; cursor += 1
    
    while state != SE and char != '\0':
        state = next_state(state, char)
        char  = buffer[cursor]; cursor += 1
    
    if state == S1 or state == S2:
        return True
    else:
        print_error(string, state, cursor)
        return False

assert is_uint("0")
assert is_uint("00") == False
assert is_uint("09") == False
assert is_uint("1")
assert is_uint("1325982034852000394500")
assert is_uint("13259823485239?45") == False
assert is_uint("Hello World") == False