输入边界断言：^, $

¥Syntax

^
$

¥Description

^ 断言当前位置是输入的开始。$ 断言当前位置是输入的结尾。两者都是断言，因此它们不消耗任何字符。

¥^ asserts that the current position is the start of input. $ asserts that the current position is the end of input. Both are assertions, so they don't consume any characters.

更准确地说，^ 断言左侧的字符超出了字符串的范围；$ 断言右侧的字符超出了字符串的范围。如果设置了 m 标志，则如果左侧的字符是 行终止符 字符，则 ^ 也会匹配；如果右侧的字符是行终止符，则 $ 也会匹配。

¥More precisely, ^ asserts that the character to the left is out of bounds of the string; $ asserts that the character to the right is out of bounds of the string. If the m flag is set, ^ also matches if the character to the left is a line terminator character, and $ also matches if the character to the right is a line terminator.

除非设置了 m 标志，否则 ^ 和 $ 断言仅在放置在模式边界时才有意义，因为它们左侧或右侧的任何其他字符必然会导致断言失败。

¥Unless the m flag is set, the ^ and $ assertions only make sense when placed at the boundaries of the pattern, because any other characters to the left or right of them would necessarily cause the assertion to fail.

y 标志不会改变这些断言的含义 - 另请参见 固定的粘性标志。

¥The y flag doesn't change the meaning of these assertions — see also anchored sticky flag.

¥Examples

¥Removing trailing slashes

以下示例从 URL 字符串中删除尾部斜杠：

¥The following example removes trailing slashes from a URL string:

function removeTrailingSlash(url) {
  return url.replace(/\/$/, "");
}

removeTrailingSlash("https://example.com/"); // "https://example.com"
removeTrailingSlash("https://example.com/docs/"); // "https://example.com/docs"

¥Matching file extensions

以下示例通过匹配文件扩展名（始终位于字符串末尾）来检查文件类型：

¥The following example checks file types by matching the file extension, which always comes at the end of the string:

function isImage(filename) {
  return /\.(?:png|jpe?g|webp|avif|gif)$/i.test(filename);
}

isImage("image.png"); // true
isImage("image.jpg"); // true
isImage("image.pdf"); // false

¥Matching entire input

有时你想确保正则表达式匹配整个输入，而不仅仅是输入的子字符串。例如，如果你要确定字符串是否是有效的 identifier，则可以将输入边界断言添加到模式的两端：

¥Sometimes you want to make sure that your regex matches the entire input, not just a substring of the input. For example, if you are determining if a string is a valid identifier, you can add input boundary assertions to both ends of the pattern:

function isValidIdentifier(str) {
  return /^[$_\p{ID_Start}][$_\p{ID_Continue}]*$/u.test(str);
}

isValidIdentifier("foo"); // true
isValidIdentifier("$1"); // true
isValidIdentifier("1foo"); // false
isValidIdentifier("  foo  "); // false

该函数在进行 codegen（使用代码生成代码）时很有用，因为你可以使用与其他字符串属性不同的有效标识符，例如 点符号 而不是 括号表示法：

¥This function is useful when doing codegen (generating code using code), because you can use valid identifiers differently from other string properties, such as dot notation instead of bracket notation:

const variables = ["foo", "foo:bar", "  foo  "];

function toAssignment(key) {
  if (isValidIdentifier(key)) {
    return `globalThis.${key} = undefined;`;
  }
  // JSON.stringify() escapes quotes and other special characters
  return `globalThis[${JSON.stringify(key)}] = undefined;`;
}

const statements = variables.map(toAssignment).join("\n");

console.log(statements);
// globalThis.foo = undefined;
// globalThis["foo:bar"] = undefined;
// globalThis["  foo  "] = undefined;

¥See also

• 断言 指南
• 正则表达式
• 字边界断言：\b、\B
• 前瞻断言：(?=...)、(?!...)
• 后向断言：(?<=...)、(?<!...)