def __init__(self, expression=None, **kwargs):
super(RegexProperty, self).__init__(**kwargs)<EOL>actual_re = expression or self.expression<EOL>if actual_re is None:<EOL><INDENT>raise ValueError('<STR_LIT>')<EOL><DEDENT>self.expression = actual_re<EOL>
Initializes new property with an expression.

:param str expression: regular expression validating this property