正则表达式-超前断言 [英] Regex - lookahead assertion
问题描述
我对前瞻性断言(?=)有问题.例如，我有表达:
I have problem with lookahead assertion (?=). For example, I have expression:
/Win(?=2000)/
如果表达式类似于Win2000，Win2000fgF，则匹配Win.我有下一个表情:
It match Win, if expression is like Win2000, Win2000fgF. I have next expression:
^(?=.*\d)(?=.*[a-z]).*$
它与数字和小写字母匹配，例如:45dF，4Dd.但是我不知道为什么它能工作并匹配所有字符:)我没有字符，在(?=.*\d)之前.我认为，只有这种表达方式才能起作用:
It match for digit and lower case letter, for example: 45dF, 4Dd. But I don't know, why it works and match all characters :) I haven't characters, which are before (?=.*\d). I think, only this expression should work:
^.\*(?=.*\d)(?=.*[a-z]).*$
(表达式前带有\*).
你能解释一下吗?
推荐答案
假设我们是正则表达式引擎，并将正则表达式^(?=.*\d)(?=.*[a-z]).*$应用于字符串2a.
Let's say we are the regex engine and apply the regex ^(?=.*\d)(?=.*[a-z]).*$ to the string 2a.
从位置0开始(在第一个字符之前):
Starting at position 0 (before the first character):
-
^:确保我们在字符串的开头:确定 -
(?=:让我们检查以下正则表达式是否可以匹配... -
.*:匹配任意数量的字符->2a.好吧. -
\d:不，我们已经结束了.让我们返回一个字符:a->不，不匹配.返回另一个:2-> MATCH！ -
):前瞻结束，匹配成功.我们静止在位置0！ -
(?=:让我们检查以下正则表达式是否可以匹配... -
.*:匹配任意数量的字符->2a.好吧. -
[a-z]:不，我们已经结束了.让我们返回一个字符:a-> MATCH！ -
):前瞻结束，匹配成功.我们静止在位置0！ -
.*:匹配任意数量的字符->2a->匹配！ -
$:让我们看看-我们在字符串的末尾吗?是的我们是！ ->比赛！ - 嘿，我们已经到了正则表达式的结尾.伟大的.比赛结束！
^: Make sure we're at the start of the string: OK(?=: Let's check if the following regex could match....*: match any number of characters ->2a. OK.\d: Nope, we're already at the end. Let's go back one character:a--> No, doesn't match. Go back another one:2--> MATCH!): End of lookahead, match successful. We're still at position 0!(?=: Let's check if the following regex could match....*: match any number of characters ->2a. OK.[a-z]: Nope, we're already at the end. Let's go back one character:a--> MATCH!): End of lookahead, match successful. We're still at position 0!.*: match any number of characters ->2a--> MATCH!$: Let's see - are we at the end of the string? Yes, we are! --> MATCH!- Hey, we've reached the end of the regex. Great. Match completed!
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