If by "preprocessing" the string you mean hoping that the input contains, say, "LATIN CAPITAL LETTER U WITH GRAVE" instead of U and "COMBINING GRAVE ACCENT", it is an unacceptable luck-based strategy. Moreover, in practice both alternatives would be implemented in very similar ways (understanding tables of how to adjust the accent for a given letter vs. understanding tables of how the accented letter is decomposed into two glyphs): it's a safe assumption that a "lightweight" library does neither.