使用 javascript 正则表达式进行正向预测
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Positive lookahead with javascript regex
提问by user3718914
I have been messed up with regex.. I found it difficult for me..I have seen a code like:
我被正则表达式搞砸了..我发现这对我来说很困难..我见过这样的代码：
function myFunction() {
var str = "Is this all there is";
var patt1 = /is(?= all)/;
var result = str.match(patt1);
document.getElementById("demo").innerHTML = result;
}
When i run this code it gave me the output is.
当我运行此代码时，它给了我输出is。
But when i add like /is(?=there)/it didnt output anything. I am new to regular expression ..hope you guys can help in in understanding positive lookahead in regex..I have followed many tutorials it didnt helped me.
但是当我添加 like/is(?=there)/它没有输出任何东西时。我是正则表达式的新手。
Hope you guys can help me out. Thanks!
希望大家能帮帮我。谢谢！
回答by zx81
The regex is(?= all)matches the letters is, but only if they are immediatelyfollowed by the letters all
正则表达式is(?= all)匹配字母is，但前提是它们后紧跟字母all
Likewise, the regex is(?=there)matches the letters is, but only if they are immediatelyfollowed by the letters there
同样，正则表达式is(?=there)匹配字母is，但前提是它们后紧跟字母there
If you combined the two in is(?= all)(?=there), you are trying to match the letters is, but only if they are immediatelyfollowed both by the letters allANDthe letters thereat the same time... which is not possible.
如果将两者结合在一起is(?= all)(?=there)，则您正在尝试匹配字母is，但前提是它们后紧跟同时是字母all和字母there......这是不可能的。
If you want to match the letters is, but only if they are immediatelyfollowed eitherby the letters allorthe letters there, then you can use:
如果你想匹配的字母is，但只有当他们被立即跟随要么由字母all或字母there，那么你可以使用：
is(?= all|there)
is(?= all|there)
If, on the other hand, you want to match the letters is, but only if they are immediatelyfollowed by the letters all there, then you can just use:
另一方面，如果您想匹配字母is，但前提是它们后紧跟字母all there，那么您可以使用：
is(?= all there)
is(?= all there)
What if I want isto be followed by alland there, but anywhere in the string?
如果我想跟is在alland 之后there，但在字符串中的任何地方怎么办？
Then you can use something like is(?=.* all)(?=.*there)
然后你可以使用类似的东西 is(?=.* all)(?=.*there)
The key to understanding lookahead
理解前瞻的关键
The key to lookarounds is to understand that the lookahead is an assertion that checks that something follows, or precedes at a specific position in the string. That is why I bolded immediately. The following article should dispel any confusion.
环视的关键是理解环视是一个断言，它检查字符串中特定位置后面或前面的内容。这就是为什么我立即加粗。下面的文章应该可以消除任何混淆。
Reference
参考
回答by hwnd
The Positive Lookaheadfails for the fact that theredoes not immediately follow is.
在正向前查找失败的，事实there不会立即跟随的。
is(?=there) # matches is when immediately followed by there
To match isif therefollows somewhere in the string, you would do:
匹配是ifthere跟在字符串的某个地方，你会这样做：
is(?=.*there)
Explanation:
说明：
is # 'is'
(?= # look ahead to see if there is:
.* # any character except \n (0 or more times)
there # 'there'
) # end of look-ahead
See Demo
看演示
A detailed tutorial I recommend: How to use Lookaheads and Lookbehinds in your Regular Expressions
我推荐的详细教程：如何在正则表达式中使用 Lookaheads 和 Lookbehinds
