Collate declared SQL variable

I've been looking at a query that looks for non-ASCII characters, reproduced below…

select line,
  patindex('%[^ !-~]%' COLLATE Latin1_General_BIN, Line) as [Position],
  substring(Line, patindex('%[^ !-~]%' COLLATE Latin1_General_BIN, Line), 1) as [InvalidCharacter],
  ascii(substring(line, patindex('%[^ !-~]%' COLLATE Latin1_General_BIN, Line), 1)) as [ASCIICode]
from staging.APARMRE1
where patindex('%[^ !-~]%' COLLATE Latin1_General_BIN, Line) > 0

…and it just strikes me that I'd want to declare a variable for '%[^ !-~]%' COLLATE Latin1_General_BIN instead of writing it out every time, but

declare @regex varchar(20) = '%[^ !-~]%' COLLATE Latin1_General_BIN;

select line,
  patindex(@regex, Line) as [Position],
  substring(Line, patindex(@regex, Line), 1) as [InvalidCharacter],
  ascii(substring(line, patindex(@regex, Line), 1)) as [ASCIICode]
from staging.APARMRE1
where patindex(@regex, Line) > 0

just doesn't do the same thing. Am I just missing some syntax? Is it impossible?

It is normal. When you create a variable it takes default collation for database.

DECLARE @regex varchar(20) = '%[^ !-~]%' COLLATE Latin1_General_BIN;

Your string with COLLATE Latin1_General_BIN is implicitly casted to string with your database default collation.

---

For example database is Case-Insensitive. I use your syntax to create case-sensitive one and check metadata of it:

DECLARE @v1 varchar(100) = 'ABC' COLLATE Latin1_General_CS_AS;

SELECT name, collation_name
FROM sys.dm_exec_describe_first_result_set(
    N'SELECT @v1 AS [@v1]', N'@v1 varchar(100)', 0);

LiveDemo

Output:

╔══════╦══════════════════════════════╗
║ name ║        collation_name        ║
╠══════╬══════════════════════════════╣
║ @v1  ║ SQL_Latin1_General_CP1_CI_AS ║
╚══════╩══════════════════════════════╝

Variables(excluding columns in table variables) do not allow to define collation so there is no syntax like:

DECLARE @v1 varchar(100) COLLATE Latin1_General_CS_AS = 'ABC' ;
-- Incorrect syntax near the keyword 'COLLATE'.