T Diagram

Cpython

Python    字节码
        C

as

C    汇编
   B

可以产生 Python 的新型编译器

Python    字节码
      汇编

rust   riscv-elf
   c-rustc  | c    x86-elf
                x86-gcc

rust  riscv-elf
   x86-elf

在 x86 平台上用 gcc 编译 c 写的目标机器为 riscv 的 rustc 项目，得到的执行文件，可以直接在 x86 平台上将 rust 代码交叉编译为 riscv 平台上运行的程序。

We use
the symbol ε (epsilon) to denote the empty string,
and we define the metasymbol ε
(boldface ε) by setting L(ε) = {ε}.

whose language is the empty set, which we
write as {}.
We use the symbol Φ for this, and we write L(Φ) = {}.

Note the difference
between {} and {ε}:
the set {} contains no strings at all,
while the set {ε} contains the
single string consisting of no characters.

r|s is the union of the languages of r and s,
or L{r|s) = L{r) U L(s) = {r, s}

S1 = {aa,b} and S2 = {a, bb],
then S1S2 = {aaa, aabb, ba, bbb}.

L(r1r2 . . . rn) = L(r1)L(r2) . . . L{rn) =
the set of strings formed by concatenating all
strings from each of L(r1),..., L(rn).

• Kleene closure: r* L(r*) = L(r)*
• 数学运算 * 比 + 优先级高
• * is given the highest precedence, concatenation is given the next highest, and | is given the lowest

(r*|s*)* = (r|s)* [^abc]

any character that is not a or b or c

• ~(ab)

b*(a*~(a|b)b*)*a*