In practice, this hypothesis producer can propose many spurious candidates w for an informal word w. As such, after we replace w by w in the hypothesis, we require that some 4-gram containing w and its surrounding words in the hypothesis appears in a formal corpus.
for example, given the un-normalized English test message "yeah must sign up , im in lt25", our English-Chinese MT system translated it into "对[yeah] 必须[must] 签 署[sign up] ， im 在[in] lt25" our normalization decoder normalized it into "yeah must sign up , i 'm in lt25 ."