(?-imsx:^REGISTER|INVITE|BYE\s\w\.\w\sSIP/2.0)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
  ^                        the beginning of the string
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  REGISTER                 'REGISTER'
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 |                        OR
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  INVITE                   'INVITE'
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 |                        OR
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  BYE                      'BYE'
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  \s                       whitespace (\n, \r, \t, \f, and " ")
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  \w                       word characters (a-z, A-Z, 0-9, _)
----------------------------------------------------------------------
  \.                       '.'
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  \w                       word characters (a-z, A-Z, 0-9, _)
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  \s                       whitespace (\n, \r, \t, \f, and " ")
----------------------------------------------------------------------
  SIP/2                    'SIP/2'
----------------------------------------------------------------------
  .                        any character except \n
----------------------------------------------------------------------
  0                        '0'
----------------------------------------------------------------------