Escaping Backslash Not Working When Using Parameter To Set Regular Expressions In Custom SQL (Google BigQuery)

Escaping a backslash in a parameter does not work as expected when regular expression is used in custom SQL in Google BigQuery.

For example:
 

-Sample data [Table]:
Column
aa
a.

-Custom SQL:
select * from Table where regexp_contains(Column,r<parameter>)

-Parameter:
a\.

-Expected result:
Column
a.

-Actual result:
No record returned

*If set parameter as 'a.' without escaping backslash, then the result will be 
Column
aa
a.

Cause

This is due to the regular expression syntax used In Google BigQuery as shown below:

REGEXP_CONTAINS(sample, r"@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+")
https://cloud.google.com/bigquery/docs/reference/standard-sql/functions-and-operators#regexp_contains

Please note there is a 'r' in front of the regular expression.
However, when using a parameter to set regular expression, escaping backslash is not working as expected.
If remove the 'r', then escaping backslash is working as expected.