number     ~ sign_maybe digit_many e
           | sign_maybe digit_any '.' digit_many e_maybe
           | sign_maybe digit_many e_maybe
           | sign_maybe non_zero digit_any

empty      ~
sign_maybe ~ [+-] | empty
digit      ~ [0-9]
non_zero   ~ [1-9]
digit_any  ~ digit*
digit_many ~ digit+
e          ~ [Ee] sign_maybe digit_many
e_maybe    ~ e | empty