1. A computer-implemented method for selecting an advertisement, the method comprising: receiving, by a processor, a search query; identifying, by a processor, a landing page that is related to one or more documents returned in response to the search query, the landing page comprising content relevant to an advertisement; determining that one or more words in the landing page also appear in an ad vector representing the advertisement; constructing a text span window as a candidate region for each of the said one or more words, of the one or more text span windows comprises a text span window vector; determining, by a processor, a vector similarity between each of the plurality of text span window vectors and the ad vector; and when the similarity in a candidate region and the ad vector is above a predetermined threshold: selecting, by a processor, the candidate region; extracting terms from the selected candidate regions; and augmenting selection of the advertisement by indexing the advertisement using the extracted terms.