Indexof not working as expected

[Karanpreet]

i have a scenario where i am adding a condition of checking a string from system property. 

 in property i added tables name the indexof needs to validate the table string i am checking should not be in property .

so if I am checking indexof only  change_task on the property .

 the index of is considering  all present in the property , since index only checks substring  from these tables change_proposal_approval,change_task,change_approval,. & Change is substring common in all of them.

 Is there a better way to do this logic

[Mark Manders]

In this case you can't use it. Same will be an issue when you exclude 'incident_task' and look for 'incident'. 

What's the use case? Isn't it easier to include the tables instead of excluding them?