Check XEN CPUIDs for vailidity. I didn't end up using this code, so here it is under the WTFPL.

xen_cpuid_verify.py

import re
import unittest

def IsValidXenCPUID(cpuid_mask):
  # host(,[0-9a-z]+=([a-z]|[0-9])+)*
  libxl_syntax = re.compile(r"""
    host              #required header
    (                 #plus many of the following:
      ,               #comma separator
      [0-9a-z]+       #field name
      =               #set to
      ([a-z]|[0-9]+)  #a single character or a numeric value
    )*
    $                 #and nothing else
    """, re.VERBOSE | re.MULTILINE | re.IGNORECASE)
  xend_fudge_item_re = r"""
    e[a-d]x=          #register of the call return value to fudge
    (
      0x[0-9A-F]{1,8} #either hardcode the reply
      | [10xks]{32}   #or edit the bitfield directly
    )
  """
  xend_string_item_re = r"""
    (                 #leafnum (the contents of EAX before the call)
      \d+(,\d+)? |    #in number format (or number,number - see [0])
      0x[0-9A-F]{1,8} #or in hex format
    )
    :                 #separator; no whitespace here
    %s                #one fudge
    (
      \s*             #whitespace between n-th fudge and separator
      ,               #comma separator
      \s*             #whitespace between seperator and n+1-th separator
      %s              #another fudge
    )*                #repeated multiple times
  """ % (xend_fudge_item_re, xend_fudge_item_re)
  # [0]: http://zhigang.org/wiki/XenCPUID#cpuid-overridden-for-xen-guests
  xend_syntax = re.compile(r"""
    \s*               #whitespace at the very start
    \[                #a list of
      \s*             #whitespace inside the list, before first element
      '%s'            #string elements
      (
        \s*           #whitespace between n-th item and separator
        ,             #separator
        \s*           #whitespace between separator and n+1-th item
        '%s'
      )*              #repeated multiple times
      \s*             #whitespace inside the list, after last element
    \]                #end of list
    \s*               #whitespace after the list
    $                 #and nothing else
    """ % (xend_string_item_re, xend_string_item_re),
    re.VERBOSE | re.MULTILINE | re.IGNORECASE)
  return (
    not cpuid_mask                    # nothing to validate
    or libxl_syntax.match(cpuid_mask) # it is valid libxl syntax
    or xend_syntax.match(cpuid_mask)  # it is valid xend syntax
  )


class TestXenCpuidSyntaxVerify(unittest.TestCase):

  def TestLibxlSyntax():
    """Accept Libxl syntax example from documentation.

    Test sources:
    * http://xenbits.xen.org/docs/unstable/man/xl.cfg.5.html"""

    self.assertTrue(IsValidXenCPUID(
      "host,tm=0,sse3=0"
    ))

  def TestXendSyntax():
    """Accept Libxl syntax example from documentation.

    Test sources:
    * http://xenbits.xen.org/docs/unstable/man/xl.cfg.5.html
    * http://zhigang.org/wiki/XenCPUID#cpuid-overridden-for-xen-guests
    * https://code.google.com/p/ganeti/issues/detail?id=159"""

    #Base example
    self.assertTrue(hv_xen.IsValidXenCPUID(
      "['1:ecx=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx0']"
    ))

    #More whitespace
    self.assertTrue(hv_xen.IsValidXenCPUID(
      "[ '1:ecx=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx0,edx=xx0xxxxxxxxxxxxxxxxxxxxxxxxxxxxx' ]"
    ))

    #MORE whitespace
    self.assertTrue(hv_xen.IsValidXenCPUID(
      """[ '1:ecx=xxxxxxxxxxx00xxxxxxxxxxxxxxxxxxx,
              eax=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx' ]"""
    ))

    #Number, number leafnum format.
    self.assertTrue(hv_xen.IsValidXenCPUID(
      """['1:edx=xxx1xxxxxxxxxxxxxxxxxxxxxxxxxxxx,
             ebx=xxxxxxxx00010000xxxxxxxxxxxxxxxx',
        '4,0:eax=001111xxxxxxxxxxxxxxxxxxxxxxxxxx']"""
    ))

    #hexadecimal leafnum format
    self.assertTrue(hv_xen.IsValidXenCPUID(
      """['1:ebx=xxxxxxxx00001010xxxxxxxxxxxxxxxx,
             edx=xxx1xxxxxxxxxxxxxxxxxxxxxxxxxxxx',
 '0x80000001:ecx=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx1x',
 '0x80000008:ecx=xxxxxxxxxxxxxxxxxxxxxxxxxx001001']"""
    ))

    # Hex fudging
    # This is found on the zhigang file, but it is actually wrong. The bitfields
    # only have 31 bits in them. The patch in bug 159 by furlongm fixes this by
    # adding one extra 'x' at the start of the bitfield, bringing it to 32 bits.
    self.assertFalse(hv_xen.IsValidXenCPUID(
      """[ '0:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
           '1:eax=0x06b1,
              ecx=xxxxxxxxxx0000xx00xxx0000000xx0,
              edx=xx00000xxxxxxx0xxxxxxxxx0xxxxxx',
           '4:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
  '0x80000000:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0']"""
    ))

    #This is the corrected version.
    self.assertTrue(hv_xen.IsValidXenCPUID(
      """[ '0:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
           '1:eax=0x06b1,
              ecx=xxxxxxxxxxx0000xx00xxx0000000xx0,
              edx=xxx00000xxxxxxx0xxxxxxxxx0xxxxxx',
           '4:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
  '0x80000000:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0']"""
    ))