awawano-useless-backreference
禁止无效的正则表达式反向引用
为什么
在JavaScript的正则表达式中，定义一个反向引用到属于模式的另一个可选部分的组，或者定义一个反向引用到出现在反向引用之后的组，或者定义一个反向引用到包含该反向引用的组，或者定义一个反向引用到位于负向前瞻中的组，这些都是语法上有效的。然而，根据规范，无论反向引用和组出现在哪种上下文中，这些情况下的反向引用总是只匹配零长度（空字符串）。
总是成功匹配零长度且不能匹配其他任何东西的反向引用是无用的。它们基本上被忽略了，可以在不改变正则表达式行为的情况下被移除。
无效的反向引用可能是代码中的错误。它通常表示正则表达式并未按照预期工作。
建议
如果反向引用总是匹配零长度，就干掉它。
错误示例
js
/^(?:(a)|\1b)$/; // reference to (a) into another alternative
/^(?:(a)|b(?:c|\1))$/; // reference to (a) into another alternative
/^(?:a|b(?:(c)|\1))$/; // reference to (c) into another alternative
/\1(a)/; // forward reference to (a)
RegExp('(a)\\2(b)'); // forward reference to (b)
/(?:a)(b)\2(c)/; // forward reference to (c)
/\k<foo>(?<foo>a)/; // forward reference to (?<foo>a)
/(?<=(a)\1)b/; // backward reference to (a) from within the same lookbehind
/(?<!(a)\1)b/; // backward reference to (a) from within the same lookbehind
new RegExp('(\\1)'); // nested reference to (\1)
/^((a)\1)$/; // nested reference to ((a)\1)
/a(?<foo>(.)b\1)/; // nested reference to (?<foo>(.)b\1)
/a(?!(b)).\1/; // reference to (b) into a negative lookahead
/(?<!(a))b\1/; // reference to (a) into a negative lookbehind/^(?:(a)|\1b)$/; // reference to (a) into another alternative
/^(?:(a)|b(?:c|\1))$/; // reference to (a) into another alternative
/^(?:a|b(?:(c)|\1))$/; // reference to (c) into another alternative
/\1(a)/; // forward reference to (a)
RegExp('(a)\\2(b)'); // forward reference to (b)
/(?:a)(b)\2(c)/; // forward reference to (c)
/\k<foo>(?<foo>a)/; // forward reference to (?<foo>a)
/(?<=(a)\1)b/; // backward reference to (a) from within the same lookbehind
/(?<!(a)\1)b/; // backward reference to (a) from within the same lookbehind
new RegExp('(\\1)'); // nested reference to (\1)
/^((a)\1)$/; // nested reference to ((a)\1)
/a(?<foo>(.)b\1)/; // nested reference to (?<foo>(.)b\1)
/a(?!(b)).\1/; // reference to (b) into a negative lookahead
/(?<!(a))b\1/; // reference to (a) into a negative lookbehind正确示例
js
/^(?:(a)|(b)\2)$/; // reference to (b)
/(a)\1/; // reference to (a)
RegExp('(a)\\1(b)'); // reference to (a)
/(a)(b)\2(c)/; // reference to (b)
/(?<foo>a)\k<foo>/; // reference to (?<foo>a)
/(?<=\1(a))b/; // reference to (a), correctly before the group as they're in the same lookbehind
/(?<=(a))b\1/; // reference to (a), correctly after the group as the backreference isn't in the lookbehind
new RegExp('(.)\\1'); // reference to (.)
/^(?:(a)\1)$/; // reference to (a)
/^((a)\2)$/; // reference to (a)
/a(?<foo>(.)b\2)/; // reference to (.)
/a(?!(b|c)\1)./; // reference to (b|c), correct as it's from within the same negative lookahead
/(?<!\1(a))b/; // reference to (a), correct as it's from within the same negative lookbehind/^(?:(a)|(b)\2)$/; // reference to (b)
/(a)\1/; // reference to (a)
RegExp('(a)\\1(b)'); // reference to (a)
/(a)(b)\2(c)/; // reference to (b)
/(?<foo>a)\k<foo>/; // reference to (?<foo>a)
/(?<=\1(a))b/; // reference to (a), correctly before the group as they're in the same lookbehind
/(?<=(a))b\1/; // reference to (a), correctly after the group as the backreference isn't in the lookbehind
new RegExp('(.)\\1'); // reference to (.)
/^(?:(a)\1)$/; // reference to (a)
/^((a)\2)$/; // reference to (a)
/a(?<foo>(.)b\2)/; // reference to (.)
/a(?!(b|c)\1)./; // reference to (b|c), correct as it's from within the same negative lookahead
/(?<!\1(a))b/; // reference to (a), correct as it's from within the same negative lookbehind