Matches strings that are Well-formed-formulas in Propositional with the following symbols {⊤,⊥,∧,∨,¬,→,↔} This matches with formulas that are looser then defined by formal syntax (See https://en.wikipedia.org/wiki/Well-formed_formula#Propositional_calculus). Namely, it's able to match with > No External Parenthesis > Generalized Conjunction, Disjunction, & Biconditional