When editor is not in incremental search state, IncSearchStr will be empty. 

When editor is in incremental search state, IncSearchStr indicates last found text. Each time user presses any character key incremental search engine looks for a new IncSearchStr in the editor text (using IncSearchBack and IncSearchIgnoreCase). Is search is successful IncSearchStr is changed and OnIncSearchChange event handler is called. 

To turn on incremental search state use IncSearchStart method or smIncrementalSearch command. To turn off incremental search state use IncSearchStop method. Editor exits incremental search state when user presses non character key (except Backspace) or clicks mouse within editor.

TCustomSyntaxMemo Properties