在正则表达式中，如果第一个括号不匹配，在JavaScript中使用replace（）和在Ruby中使用gsub时，$ 1是否为空字符串？

Using JavaScript, to extract the prefix foo. including the . from foo.bar, I could use:

使用JavaScript,提取前缀foo。包括。从foo.bar,我可以使用:

> "foo.bar".replace(/(\w+.)(.*)/, "$1")
"foo."

But if there is no such prefix, I'd expect it to give an empty string or null, but instead it gives the full string:

但是如果没有这样的前缀,我希望它给出一个空字符串或null,而是它给出完整的字符串:

> "foobar".replace(/(\w+.)(.*)/, "$1")
"foobar"

Why will $1 give the whole string? -- as I thought it means the first parenthesis.

为什么1美元会给整个字符串? - 因为我认为这意味着第一个括号。

Maybe it means the first parenthesis that actually matched?

也许这意味着实际匹配的第一个括号?

If #1 is true, then maybe a common, standard technique is to use ?, which works in Ruby:

如果#1为真,那么可能使用一种常见的标准技术?,它适用于Ruby:

using irb:
```
> "foo.bar".gsub(/(\w+\.)?(.*)/, '\1')
"foo."
> "foobar".gsub(/(\w+\.)?(.*)/, '\1')
""
```
Because the ? is optional, and it will match anyway. However, it doesn't work in JavaScript:

因为?是可选的,无论如何它都会匹配。但是,它在JavaScript中不起作用:
```
> "foobar".replace(/(\w+.)?(.*)/, "$1")
"foobar"
```
I can use match() in JavaScript to do it, and it will be quite clean, but just for the sake of understanding replace() more:

我可以在JavaScript中使用match()来完成它,它会很干净,但只是为了理解replace()更多:
What is the reason that it works differently in Ruby vs JavaScript, and do #1 and #2 above also apply and/or what is a good alternative way to "grab" the prefix or get "" if it doesn't exist using replace()?

它在Ruby与JavaScript中的工作方式不同的原因是什么,以及上面的#1和#2是否也适用和/或什么是“抓住”前缀或如果使用替换不存在则获取“”的好方法()?

2 个解决方案

#1

FYI, I think your JavaScript's regex isn't correct since it doesn't escape the . (dot) character.

仅供参考,我认为你的JavaScript的正则表达式是不正确的,因为它没有逃脱。 (点)字符。

The reason why $1 returns the whole string is $1 tricked you to believe that it matches the first group (which isn't true).

$ 1返回整个字符串的原因是$ 1诱骗你相信它匹配第一组(这不是真的)。

/* your js regex is /(\w+.)/, I use /(\w+\.)/ instead to demonstrate it */
"foobar".replace(/(\w+\.)/, "$1"); // 'foobar'

It's because $1 matches nothing which is (empty) then the regex tries to replace the original string foobar with $1 (since it doesn't match anything it just returns the whole original string. To make it clears take a look at following example.

这是因为$ 1匹配任何(空)然后正则表达式尝试用$ 1替换原始字符串foobar(因为它不匹配任何它只返回整个原始字符串。为了使它清除,请看下面的示例。

"foobar".replace(/(\w+\.)/, '-');    // 'foobar' (No matches, so nothing get replaced)
"foobar".replace(/(\w+\.)/, '$1');   // 'foobar' (No matches, $1 is empty, nothing get replaced)
"foobar.a".replace(/(\w+\.)/, '-');  // '-a' (matches 'foobar.' so replaces 'foobar.' with '-') + ('a')
"foobar.a".replace(/(\w+\.)/, '$1'); // 'foobar.a' (matches 'foobar.' so replaces 'foobar.' with itself) + ('a')

#2

The replace method in JavaScript gives you a copy of the original string whether you successfully altered it or not.

JavaScript中的replace方法为您提供原始字符串的副本,无论您是否成功更改了它。

So for instance:

例如:

alert( "atari.teenageRiot".replace(/5/,'reverse polarity of the neutron flow') );
//"atari.teenageRiot"

Replace isn't about finding matches. It's about altering the string by replacing what you match with the first argument with the second argument so you always return the string that's meant to be changed whether it was changed or not.

替换不是寻找匹配。它是通过用第二个参数替换第一个参数所匹配的字符串来改变字符串,因此无论是否更改,您总是返回要更改的字符串。

Also, I would use this instead:

另外,我会改用它:

"foo.bar".replace(/(\w+\.)(.*)/, "$1")

You didn' have the \ before . so it was treated as the wild card which matches most characters.

你之前没有过。所以它被视为匹配大多数字符的外卡。

#1