[sqlalchemy] How can I use negate for operator in UserDefinedType.Comparator

[Frazer McLean]

I am trying to add regexp and iregexp operators to a subclass of the Text 
type. I'm using SQLAlchemy v1.0.12.

I am able to correctly use '~' and '!~' operators if I use a notregexp 
method like this:

import sqlalchemy.types as types

class TEXT(types.Text):
    class Comparator(types.Text.Comparator):
        def regexp(self, other):
            return self.op('~')(other)

        def notregexp(self, other):
            return self.op('!~')(other)

    comparator_factory = Comparator

However, I would like to correctly emit the '!~' operator if I use not_ or ~ 
in SQLAlchemy, so I tried to do the following:

import sqlalchemy.types as types
from sqlalchemy.sql.expression import BinaryExpression
from sqlalchemy.sql import operators

class TEXT(types.Text):
    class Comparator(types.Text.Comparator):
        def regexp(self, other):
            return BinaryExpression(
                self, other, operator=operators.custom_op('~'), type_=TEXT,
                negate=operators.custom_op('!~'))

    comparator_factory = Comparator

But I can't seem to get that to work. I was attempting to copy the 
UnaryExpression example here 
<http://docs.sqlalchemy.org/en/latest/core/custom_types.html?highlight=userdefinedtype#redefining-and-creating-new-operators>
.

I see the patch on BitBucket 
<https://bitbucket.org/zzzeek/sqlalchemy/issues/1390/postgresql-regular-expression-operators>
 
for adding these operators, but I'm trying to do the same with the public 
API. Is it possible?

Thanks,

Frazer McLean

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