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Currently the it's O(n) where n is the length of the text inside the JTextPane. You could reduce it if you process only the modified regions of the document in the event listener. I think DocumentEvent's getLength() and getOffset() method could help here.

Another notes:

1. int index = 0, carriage = 0;
   

I'd put the variable declarations to separate lines. From Code Complete, 2nd Edition, p759:

With statements on their own lines, the code reads from top to bottom, instead of top to bottom and left to right. When you’re looking for a specific line of code, your eye should be able to follow the left margin of the code. It shouldn’t have to dip into each and every line just because a single line might contain two statements.

2. Be aware of static helper classes, like Keywords. You might find my former answer about it useful.