def __init__( self, pattern, flags=0):
        """The parameters C{pattern} and C{flags} are passed to the C{re.compile()} function as-is. See the Python C{re} module for an explanation of the acceptable patterns and flags."""
        super(Regex,self).__init__()

        if isinstance(pattern, basestring):
            if not pattern:
                warnings.warn("null string passed to Regex; use Empty() instead",
                        SyntaxWarning, stacklevel=2)

            self.pattern = pattern
            self.flags = flags

            try:
                self.re = re.compile(self.pattern, self.flags)
                self.reString = self.pattern
            except sre_constants.error:
                warnings.warn("invalid pattern (%s) passed to Regex" % pattern,
                    SyntaxWarning, stacklevel=2)
                raise

        elif isinstance(pattern, Regex.compiledREtype):
            self.re = pattern
            self.pattern = \
            self.reString = str(pattern)
            self.flags = flags
            
        else:
            raise ValueError("Regex may only be constructed with a string or a compiled RE object")

        self.name = _ustr(self)
        self.errmsg = "Expected " + self.name
        self.mayIndexError = False
        self.mayReturnEmpty = True
The parameters C{pattern} and C{flags} are passed to the C{re.compile()} function as-is. See the Python C{re} module for an explanation of the acceptable patterns and flags.
src/pkg_resources/_vendor/pyparsing.py