Most efficient: Cut off varchar after first space?

Question

What is the most efficient way to cut off a varchar after the first space i.e. given 'FIRST_STRING SECOND_STRING THIRD_STRING', it should return 'FIRST_STRING'? This will be run on potentially 100s of thousands of rows.

Answer 1

Notice the +' ' this will trap any single word

Example

Declare @S varchar(max) = 'FIRST_STRING SECOND_STRING THIRD_STRING'

Select left(@S,charindex(' ',@S+' ')-1)

Returns

FIRST_STRING