StartExpr allows defining more complex condition for starting auto-completion than using StartString property. 

StartExpr - is backward regular expression. I.e. when matching characters numerated in descending order. 

For example, it is required to test "." or "->" as start strings for starting completion. To test them next expression should be defined:

StartExpr := '\.|>-'

You may see that for testing "->" we used ">-" expression, because characters are tested in backward order. 

Auto-completion starts (with specified delay) if either StartExpr or StartString conditions are succeeded.

TAutoCompletePopup Properties