^ 和 $ 在正则表达式中是什么意思? [英] What do ^ and $ mean in a regular expression?
问题描述
"\\w+@\\w+[.]\\w+" 和 "^\\w+@\\w+[.]\\w+$ 有什么区别?我试过用谷歌搜索，但没有运气.
What is the difference between "\\w+@\\w+[.]\\w+" and "^\\w+@\\w+[.]\\w+$"? I have tried to google for it but no luck.
推荐答案
^ 的意思是匹配字符串的开头"(更准确地说，是第一个之前的位置字符串中的字符，因此它与实际字符不匹配).
^ means "Match the start of the string" (more exactly, the position before the first character in the string, so it does not match an actual character).
$ 表示匹配字符串的结尾"(字符串中最后一个字符之后的位置).
$ means "Match the end of the string" (the position after the last character in the string).
两者都被称为 anchors 并确保匹配整个字符串而不仅仅是一个子串.
Both are called anchors and ensure that the entire string is matched instead of just a substring.
因此在您的示例中，第一个正则表达式将在 email@address.com.uk 上报告匹配项，但匹配的文本将是 email@address.com，可能不是你所期望的.第二个正则表达式只会失败.
So in your example, the first regex will report a match on email@address.com.uk, but the matched text will be email@address.com, probably not what you expected. The second regex will simply fail.
小心，因为一些正则表达式实现隐式地将正则表达式锚定在字符串的开头/结尾(例如 Java 的 .matches()，如果你正在使用它).
Be careful, as some regex implementations implicitly anchor the regex at the start/end of the string (for example Java's .matches(), if you're using that).
如果设置了多行选项(例如，使用 (?m) 标志，或者通过执行 Pattern.compile("^\\w+@\\w+[.]\\w+$", Pattern.MULTILINE))，然后 ^ 和 $ 也在行的开头和结尾匹配.
If the multiline option is set (using the (?m) flag, for example, or by doing Pattern.compile("^\\w+@\\w+[.]\\w+$", Pattern.MULTILINE)), then ^ and $ also match at the start and end of a line.
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