An IPv4 address validator in Python. An example of an implementation of a regular expression.

ipv4_validator.py

#!/usr/bin/env python3

# Regular Expression:
# (0|1[0-9]{0,2}|[2-9][0-9]?|2[0-4]?[0-9]|25[0-5])
#     .(0|1[0-9]{0,2}|[2-9][0-9]?|2[0-4]?[0-9]|25[0-5])
#     .(0|1[0-9]{0,2}|[2-9][0-9]?|2[0-4]?[0-9]|25[0-5])
#     .(0|1[0-9]{0,2}|[2-9][0-9]?|2[0-4]?[0-9]|25[0-5])

import sys

INIT = 0x00
ACCEPTS = [ num for num in range(0x31, 0x3c + 1) ]
TRANS = {}

for d in range(4):
  dd = 0x10 * d

  TRANS[0x99] = {}
  for i in range(256):
    TRANS[0x99][chr(i)] = 0x99

  for s in range(0x0d):
    ss = s + dd
    TRANS[ss] = {}
    for i in range(0, 256):
      TRANS[ss][chr(i)] = 0x99

for d in range(4):
  dd = 0x10 * d

  TRANS[0x00 + dd]['0'] = 0x01 + dd
  TRANS[0x00 + dd]['1'] = 0x02 + dd
  TRANS[0x00 + dd]['2'] = 0x03 + dd
  for c in ['3', '4', '5', '6', '7', '8', '9']:
    TRANS[0x00 + dd][c] = 0x04 + dd

  for c in ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9']:
    TRANS[0x02 + dd][c] = 0x05 + dd

  for c in ['0', '1', '2', '3', '4']:
    TRANS[0x03 + dd][c] = 0x06 + dd
  TRANS[0x03 + dd]['5'] = 0x07 + dd
  for c in ['6', '7', '8', '9']:
    TRANS[0x03 + dd][c] = 0x08 + dd

  for c in ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9']:
    TRANS[0x04 + dd][c] = 0x09 + dd

  for c in ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9']:
    TRANS[0x05 + dd][c] = 0x0a + dd

  for c in ['0', '1', '2', '3', '4', '5', '6', '7', '8', '9']:
    TRANS[0x06 + dd][c] = 0x0b + dd

  for c in ['0', '1', '2', '3', '4', '5']:
    TRANS[0x07 + dd][c] = 0x0c + dd

  if dd != 0x30:
    TRANS[0x01 + dd]['.'] = 0x10 + dd
    TRANS[0x02 + dd]['.'] = 0x10 + dd
    TRANS[0x03 + dd]['.'] = 0x10 + dd
    TRANS[0x04 + dd]['.'] = 0x10 + dd
    TRANS[0x05 + dd]['.'] = 0x10 + dd
    TRANS[0x06 + dd]['.'] = 0x10 + dd
    TRANS[0x07 + dd]['.'] = 0x10 + dd
    TRANS[0x08 + dd]['.'] = 0x10 + dd
    TRANS[0x09 + dd]['.'] = 0x10 + dd
    TRANS[0x0a + dd]['.'] = 0x10 + dd
    TRANS[0x0b + dd]['.'] = 0x10 + dd
    TRANS[0x0c + dd]['.'] = 0x10 + dd

def validate(s):
  state = INIT
  for c in s:
    print(f'{state:02x} -({c})-> {TRANS[state][c]:02x}')
    state = TRANS[state][c]
  print(f'{s}: {state in ACCEPTS}({state:02x})')

if __name__ == '__main__':
  for s in sys.argv[1:]:
    validate(s)

Regular Expression: D.D.D.D (where D is (0|1[0-9]{0,2}|[2-9][0-9]?|2[0-4]?[0-9]|25[0-5]))