How to escape underscore character in PATINDEX pattern argument?

Question

I\'ve found a solution for finding the position of an underscore with PATINDEX :

DECLARE @a VARCHAR(10)  
SET     @a = \'37_21\'

PRINT PATINDEX(\'%_%\', @a)         


        

Answer 1

I've always done it with brackets: '%[_]%'

Answer 2

To match two underscores, each must be bracketed

'%[__]%' -- matches single _ with anything after

'%[_][_]%' -- matches two consecutive _

Answer 3

You can escape using the [ and ] characters like so:

PRINT PATINDEX('%[_]%', '37_21')