How do I validate the format of a MAC address?

Question

What's the best way to validate that an MAC address entered by the user?

The format is HH:HH:HH:HH:HH:HH, where each H is a hexadecimal character.

For instance, 00:29:15:80:4E:4A is valid while 00:29:804E4A is invalid.

Answer 1

If you mean just the syntax then this regexp should work for you

import re
...
if re.match("[0-9a-f]{2}([-:]?)[0-9a-f]{2}(\\1[0-9a-f]{2}){4}$", x.lower()):
    ...

it accepts 12 hex digits with either : or - or nothing as separators between pairs (but the separator must be uniform... either all separators are : or are all - or there is no separator).

This is the explanation:

• [0-9a-f] means an hexadecimal digit
• {2} means that we want two of them
• [-:]? means either a dash or a colon but optional. Note that the dash as first char doesn't mean a range but only means itself. This subexpression is enclosed in parenthesis so it can be reused later as a back reference.
• [0-9a-f]{2} is another pair of hexadecimal digits
• \\1 this means that we want to match the same expression that we matched before as separator. This is what guarantees uniformity. Note that the regexp syntax is \1 but I'm using a regular string so backslash must be escaped by doubling it.
• [0-9a-f]{2} another pair of hex digits
• {4} the previous parenthesized block must be repeated exactly 4 times, giving a total of 6 pairs of digits: <pair> [<sep>] <pair> ( <same-sep> <pair> ) * 4
• $ The string must end right after them

Note that in Python re.match only checks starting at the start of the string and therefore a ^ at the beginning is not needed.

Answer 2

I hate programs that force the user to think a like a computer.

Make it more friendly by accepting any valid format.

Strip the separator, whatever it is, then get the hex value that's left. That way if a user enters dashes or spaces it also works.

import string
allchars = "".join(chr(a) for a in range(256))
delchars = set(allchars) - set(string.hexdigits)

def checkMAC(s):
  mac = s.translate("".join(allchars),"".join(delchars))
  if len(mac) != 12:
      raise ValueError, "Ethernet MACs are always 12 hex characters, you entered %s" % mac 
  return mac.upper()


checkMAC("AA:BB:CC:DD:EE:FF")
checkMAC("00-11-22-33-44-66")
checkMAC("1 2 3 4 5 6 7 8 9 a b c")
checkMAC("This is not a mac")

Answer 3

If you want to ensure that there is either '-' or ':' throughout but not both, you can use following in Python:

import re

def is_valid_macaddr802(value):
    allowed = re.compile(r"""
                         (
                             ^([0-9A-F]{2}[-]){5}([0-9A-F]{2})$
                            |^([0-9A-F]{2}[:]){5}([0-9A-F]{2})$
                         )
                         """,
                         re.VERBOSE|re.IGNORECASE)

    if allowed.match(value) is None:
        return False
    else:
        return True