creating index with a cast statement in the predicate

Good afternoon

I am currently running postgres 9.4.8.

I would like to create an index on a column named value: defined as
varchar(255)
Values in this column can be many, including both dates and non-dates.
The developers run a query that first uses a regular expression to filter
non-dates out
then, ANDS it with the following
*and cast(value as date) >= date_trunc('MONTH', current_date) - interval '
2 Month '*
When I attempt to create an index using this statement in the
predicate, I get the error that states that functions in an index predicate
must be marked as Immutable.
Our internal functions date_trunc (all 3) and date are all already defined
as immutable.
I can create an index with the regular expression as the predicate but not
the cast statement comparison.

Here is the regular expression index I constructed:
"primets_custattr_small_regexp" btree (value) WHERE value::text ~
'([0-9][0-9][0-9][0-9])-(0[13578]|1[02])-(0[1-9]|[12][0-9]|3[01])|([0-9][0-9][0-9][0-9])-(0[469]|11])-(0[1-9]|[12][0-9]|30)|([0-9][0-9][0248][048]|[0-9][0-9][13579][26])-(02)-(0[1-9]|1[0-9]|2[0-9])|([0-9][0-9][0248][1235679]|[0-9][0-9][13579][01345789])-(02)-(0[1-9]|1[0-9]|2[0-8])'::text

I would appreciate any insight here. Thank you

--
*Mark Steben*
Database Administrator
@utoRevenue <http://www.autorevenue.com/> | Autobase
<http://www.autobase.net/>
CRM division of Dominion Dealer Solutions
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