Expletive constructions begin with "there is / are" or "it is", and their variations. These do not add any meaning, and can be easily replaced for example: There are twenty-five students who have already expressed a desire... can be easily rewritten to Twenty-five students already expressed a desire...
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Used to be /((there|it)\s(is|was|were|are|will be|wont)|(it's|there's))/i, switched to list. Not everything is suited for regex :)