Question

如果元字符?与前面的元素零或一次匹配，那么

为什么

"ab".match(/a?/)

返回["a"]，

但

"ab".match(/b?/)

返回[""]

Answer 1

因为那是第一场比赛。正则表达式首先尝试匹配位置0，其中正则表达式＃1匹配a，正则表达式＃2匹配空字符串。然后它尝试匹配位置1，其中正则表达式＃1匹配空字符串，正则表达式＃2匹配字母b。最后，它尝试匹配位置3，其中两个正则表达式都匹配空字符串。

将返回的匹配与全局标记进行比较：

> "ab".match(/a?/)
["a"]
> "ab".match(/a?/g)
["a", "", ""]
> "ab".match(/b?/)
[""]
> "ab".match(/b?/g)
["", "b", ""]

为什么不在第一种情况下返回[“”]？

由于backtracking的机制。当试图在某个位置匹配时，引擎将尝试贪婪¹测试正则表达式的所有字母与字符串的字母。当它使用该方法到达正则表达式的末尾时，匹配成功。当一个字母不适合时，它会尝试返回正则表达式以查看是否可以进行任何遗漏 - 使用*或?等修饰符 - 或替代（{{1} }）需要考虑，然后从那里继续。

示例：在|的位置0匹配/b?/：

//   - "": ✓
/b/  - "a": ×
/b?/ - "": ✓ - succeed (end of regex)
  ^ means here that the "b" token is omitted

示例：在"ab"的位置0匹配/a?/：

//  - "": ✓
/a/ - "a": ✓ - succeed (end of regex)

示例：在"ab"

的第0位匹配/ab?(bc)?/

//        - "": ✓
/a/       - "a": ✓
/ab/      - "ab": ✓
/ab(b)/   - "abc": ×
/ab(bc)?/ - "ab": ✓ - succeed (end of regex)

_{1：通常，至少。如果要控制完全匹配行为，许多正则表达式风格还提供lazy或possessive的量词。例如，"abc"与/ab??(bc)?/}

中的abc匹配

Answer 2

我想知道我是否可以通过以下类比更好地解释它。

想象一下，一个友好的快餐收银员，一群饥肠辘辘的顾客和一种装满汉堡的传送带。她一个接一个地向每个顾客提供汉堡，直到所有顾客都满意为止（如果留下一些汉堡，它并不重要）。然后，她写下每个客户的清单，并将此清单提供给她的经理。收银员将是正则表达式引擎，汉堡是输入字符串中的符号，客户是正则表达式子表达式。经理是匹配功能。

例如，当abbc与/[a]b+./匹配时，场景会像这样播放：


("[a]", "b+" and "." stand the queue)

Cashier: Hi, "[a]", would you like "a"?
[a]: Sure, thanks!
C: would you also like "b"?
[a]: No, thanks, I'm fine (goes).
C: Hi, "b+", would you like "b"?
b+: Sure.
C: Would you like another "b"?
b+: Yes, I'm hungry.
C: Can I offer you "c" also?
b+: Not my kind of thing (goes).
C: Hi, ".", I have only one "c" left.
.: I don't care what it is, just gimme it (goes).
C: All served! Looks like I'll get the job!

如果收银员无法满足顾客，她有权打电话给前一个并要求他们回馈他们所拥有的东西。这被称为＆＃34;回溯＆＃34;。考虑：


 "abx" against /.+[xyz]/

 C: Hi, ".+", would you like "a"?
 .+: Yum-yum!
 C: How about "b"?
 .+: Yum-yum!
 C: And "x"?
 .+: Yum-yum!
 C: My belt is empty! (.+ goes)
 C: Hi, "[xyz]", I'm afraid I'm sold out. 
 [xyz]: That's out of the question. Can I see the manager?
 C: Wait, I think we can sort it out! (calls ".+")
 C (to ".+"): Sorry pal, you know, this nasty guy over there... 
              I wonder if you could you give me back your last one?
 .+: No prob... (gives "x" back)
 C (to "[xyz]"): I've got something for you. Do you eat "x"?
 [xyz]: If you want to get anything done in this country 
        you've got to complain until you are blue in the mouth 
        (gets his "x" and goes in a rage)
 C: Gosh, what a day...

现在回到你的例子：


Scene I. "ab" against /a?/ 
Burgers: a and b, customer: a?

C: Hi, "a?" would you like "a"?
a?: Sure, thanks.
C: Can I offer you "b" also?
a?: No, thanks, I'm fine (goes).
Manager: I need the inventory report, now!
C: Here you go: "a?" got "a", we have "b" and "c" left.

Scene II. "ab" against /b?/ 
Burgers: a and b, customer: b?

C: Hi, "b?" would you like "a"?
b?: No thanks, but that's no problem. (goes).
M: Status?
C: "b?" got nothing and went. a, b, c are still there.

所以，基本上b?是一个非常好（而不是特别饥饿）的人，即使收银员对他没有任何帮助，他也很高兴。如果他是队列中唯一的一员，那就是她的幸运日！

为什么匹配函数只返回第一个字母？

2 个答案: