'Filter results of a list operation.
GCE supports server-side filtering of resources returned by a list
operation. Syntax of the filter expression is fully described in the
GCE API reference doc, but in brief it is::
FIELD_NAME COMPARISON_STRING LITERAL_STRING
where FIELD_NAME is the resource\'s property name, COMPARISON_STRING is
\'eq\' or \'ne\', and LITERAL_STRING is a regular expression in RE2 syntax.
>>> for sublist in l.filter(\'name eq ...-map\'):
...   sublist
[<GCEUrlMap id="..." name="cli-map">,                 <GCEUrlMap id="..." name="web-map">]
API reference: https://cloud.google.com/compute/docs/reference/latest/
RE2 syntax: https://github.com/google/re2/blob/master/doc/syntax.txt
:param  expression: Filter expression described above.
:type   expression: ``str``
:return: This :class:`GCEList` instance
:rtype:  :class:`GCEList`'

def filter(self, expression):

    self.params['filter'] = expression
    return self
