You do not need to interact with managed code, as you can use LIKE:
CREATE TABLE #Sample(Field varchar(50), Result varchar(50))
GO
INSERT INTO #Sample (Field, Result) VALUES ('ABC123 ', 'Do not match')
INSERT INTO #Sample (Field, Result) VALUES ('ABC123.', 'Do not match')
INSERT INTO #Sample (Field, Result) VALUES ('ABC123&', 'Match')
SELECT * FROM #Sample WHERE Field LIKE '%[^a-z0-9 .]%'
GO
DROP TABLE #Sample
As your expression ends with + you can go with '%[^a-z0-9 .][^a-z0-9 .]%'
EDIT:
To make it clear: SQL Server doesn’t support regular expressions without managed code. Depending on the situation, the LIKE operator can be an option, but it lacks the flexibility that regular expressions provides.