--- count(str[, str2[,  ... ]])

    ʸˤ롢str ˴ޤޤʸο֤ޤ

    str η tr(1) [manual page] ƱǤĤޤꡢ
    `a-c'  a  c ̣"^0-9" Τ褦
    ʸƬ `^' ξϻʸʳ̣ޤ

    `-' ʸξüˤʤˤϰϻΰ̣ˤʤޤ
    Ʊͤˡ`^' ⤽θ̤ʸƬˤȤǤޤ
    `-', `^', `\' ϥХåå(`\')ˤ
    ꥨפ뤳ȤǤޤ

    ʣꤷϡ٤Ƥΰѽ̣ޤ

          p 'abcdefg'.count('c')               # => 1
          p '123456789'.count('2378')          # => 4
          p '123456789'.count('2-8', '^4-6')   # => 4

    ʲϤΥ᥽åɤŵŪʻǡեιԿ֤ޤ
    (ե˲ԤɬפǤ)

          n_lines = File.open("foo").read.count("\n")

