In Tableau 9 you can use regular expressions in formulas, it makes the task simpler:
REGEXP_EXTRACT([Description], "Target Name: (.*?) ")
Alternatively in Tableau 9 you can use the new FINDNTH function:
MID(
[Description],
FINDNTH([Description]," ", 5) + 1,
FINDNTH([Description]," ", 6) - FINDNTH([Description]," ", 5) - 1
)
Prior to Tableau 9 you'd have to use string manipulation methods similar to what you've tried, just need to be very careful with arithmetic and providing the right arguments (the third argument in MIDis length, not index of the end character, so we need to subtract the index of the start character):
MID(
[Description]
, FIND([Description], "Target Name:") + 13
, FIND([Description], " ", FIND([Description], "Target Name:") + 15)
- (FIND([Description], "Target Name:") + 13)
)