1. A method, implemented on at least one computing device each of which has at least one processor, storage, and a communication platform connected to a network for determining a keyword from user agent strings, the method comprising: receiving a plurality of user agent strings; grouping the plurality of user agent strings into one or more clusters, wherein the one or more clusters comprise a first cluster that includes two or more user agent strings; comparing the two or more user agent strings in the first cluster; extracting a longest common subsequence among the two or more user agent strings; removing the longest common subsequence from each user agent string to obtain a remaining subsequence; and determining a keyword from the first cluster based on at least one of the longest common subsequence and the remaining subsequence, wherein the keyword represents a type of user agent information.