Emacs 正则表达式匹配行尾后跟新行

Question

空行后跟新行的匹配对于 regex 失败，^$^J但对于 regex 匹配成功^^J。以前的正则表达式有什么问题？

Answer 1

$在正则表达式中通常匹配行尾的空字符串，但实际上，只有当 出现$在正则表达式中的特定位置时（例如，在正则表达式的末尾，或在子组的末尾，IIRC ）。如果$出现在“中间”，则它仅与该$字符匹配。同样适用于^. 例如(string-match "a^b$c" "1a^b$c2")返回 1。

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C-hig (emacs) Regexps记录此行为：

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\xe2\x80\x98^\xe2\x80\x99\n     is a special character that matches the empty string, but only at\n     the beginning of a line in the text being matched.  Otherwise it\n     fails to match anything.  Thus, \xe2\x80\x98^foo\xe2\x80\x99 matches a \xe2\x80\x98foo\xe2\x80\x99 that occurs\n     at the beginning of a line.\n\n     For historical compatibility reasons, \xe2\x80\x98^\xe2\x80\x99 can be used with this\n     meaning only at the beginning of the regular expression, or after\n     \xe2\x80\x98\\(\xe2\x80\x99 or \xe2\x80\x98\\|\xe2\x80\x99.\n\n\xe2\x80\x98$\xe2\x80\x99\n     is similar to \xe2\x80\x98^\xe2\x80\x99 but matches only at the end of a line.  Thus,\n     \xe2\x80\x98x+$\xe2\x80\x99 matches a string of one \xe2\x80\x98x\xe2\x80\x99 or more at the end of a line.\n\n     For historical compatibility reasons, \xe2\x80\x98$\xe2\x80\x99 can be used with this\n     meaning only at the end of the regular expression, or before \xe2\x80\x98\\)\xe2\x80\x99\n     or \xe2\x80\x98\\|\xe2\x80\x99.\n

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