We investigate search engines' mechanism for allocating impressions generated from different search terms. This mechanism is equivalent to running an independent GSP auction for each search term only when the number of search terms is small. In practice, the number of search terms is so large that an advertiser cannot possibly communicate to the search engine all the GSP auctions that he wishes to participate in. For example, a travel agency is interested in all search terms pertaining to flight, including "flight to boston", "ticket to SFO", "cheap airfare", etc. Therefore, the search engine introduces broad match keywords as a bidding language that allows an advertiser to submit a bid for multiple GSP auctions at once. However, with broad match keywords, the GSP auctions are no longer independent, i.e. an advertiser's bid in one auction may depend on his bid in another auction.We propose the broad match mechanism as a model that captures this aspect of the multi-keyword sponsored search mechanism. We study the performance of this mechanism under the price of anarchy (POA) framework. We identify two properties of broad match keywords, namely expressiveness and homogeneity, that characterize the POA and we prove almost tight bounds on the POA. The bounds allow us to explore trade-offs between the two properties. We introduce the exact-match-only mechanism whose performance, when compared to that of broad match mechanisms, gives us an insight into the net benefit of broad match keywords. The broad match mechanism can also be viewed as a mechanism that copes with severe communication constraint i.e. the valuation of an advertiser is described by many more numbers than the search engine can solicit.