  We study (von Neumann) regular $^*$-subalgebras of $B(H)$, which we call
R$^*$-algebras. The class of R$^*$-algebras coincides with that of
"E$^*$-algebras that are pre-C$^*$-algebras" in the sense of Z. Sz\H{u}cs and
B. Tak\'acs. We give examples, properties and questions of R$^*$-algebras. We
observe that the class of unital commutative R$^*$-algebras has a canonical
one-to-one correspondence with the class of Boolean algebras. This motivates
the study of R$^*$-algebras as that of noncommutative Boolean algebras. We
explain that seemingly unrelated topics of functional analysis, like AF
C$^*$-algebras and incomplete inner product spaces, naturally arise in the
investigation of R$^*$-algebras. We obtain a number of interesting results on
R$^*$-algebras by applying various famous theorems in the literature.
