8. At least one non-transitory computer-readable storage medium encoded with computer-executable instructions that, when executed, perform a method comprising: receiving a free-text input utterance to be recognized; applying a statistical language model to the free-text input utterance to obtain a plurality of candidate word sequences for automatic speech recognition of the input utterance, each of the plurality of candidate word sequences having a corresponding initial score generated by the statistical language model; for one or more of the plurality of candidate word sequences, detecting words corresponding to a sequence of digits, and analyzing the detected words corresponding to the sequence of digits, using at least one processor, to generate one or more hypotheses that the words corresponding to the sequence of digits belong to one or more semantic classes, and to determine a separate score for each of the one or more hypotheses indicating a probability that the words corresponding to the sequence of digits belong to the semantic class; revising the initial scores for each of the one or more plurality of candidate word sequences, wherein revising the initial score for each of the one or more plurality of candidate word sequences comprises computing a revised score for the respective candidate word sequence based, at least in part, on the separate scores determined for the one or more hypotheses generated for each of the one or more semantic classes; selecting, from among the plurality of candidate word sequences, a best-scoring candidate word sequence using the revised scores; and outputting the best-scoring candidate word sequence as an output recognized word sequence for the input utterance.
10. The at least one non-transitory computer-readable storage medium of claim 8 , wherein revising the initial score for a first candidate word sequence of the plurality of candidate word sequences comprises accounting for a probability of occurrence of the detected words corresponding to the sequence of digits in the first candidate word sequence conditioned on occurrence of a semantic class hypothesized for the detected words corresponding to the sequence of digits.