Detailed explanation of JS regular single-line mode

This time I will bring you a detailed explanation of the JS regular single-line mode. What are the precautions when using the JS regular single-line mode. The following is a practical case, let's take a look.

Regular Expression was first implemented by Ken Thompson in his improved QED editor in 1970. The simplest metacharacter in the regular expression "." matched at that time is any character except line breaks:

"." is a regular expression which matches any character except .

The above sentence comes from the official document of QED in 1970 , which may be the first regular document in history.

Why is this stipulated? This is because QED edits files in line units, and the newline character at the end of the line is also included in the content of this line. For example, if you want to delete all single-line comments in a piece of code, you can use the following command in QED:

1,$s#//.*##

If "." can match the newline character, then the newline character will also be deleted, and it will Causes these lines to be merged with the next line, which is usually not what we want. Therefore, "." was designed not to match newlines when it was originally invented. Although there is no QED command on the current operating system for us to test, we still have VIM, and the "." in VIM cannot match the newline character for the same reason.

Unlike in Node, reading filesusually reads the entire file in one go, Perl inherits the tradition of many Linux commands reading files line by line, like this:

while (<>) {print $_}

There is also a newline character at the end of

_, so Perl naturally inherits QED's rule that "." does not match newline characters. But Perl is a programming language after all, not an editor. The objects that its regular expressions need to match are not only single lines of text, but also multi-line text. Therefore, in its regular expressions, "." There is a need for cross-line matching, so Perl invented the regular single-line mode /s, which allows "." to also match newline characters. The official description of the /s modifier in Perl used to turn on single line mode is "Treat the string as single line". This "single line" should be understood like this: "." can only match in normal mode. Inline characters cannot span lines; in single-line mode, Perl will pretend to treat multi-line strings as one line, and treat the newline characters as inline characters, so "." can match them. To put it more vividly, the following three lines of text

1
2
3

are regarded as "1\n2\n3\n" one line of text. This is what single-line mode means.

But the terrible thing is that for the same reason (string variables can contain multiple lines of text), Perl also invented the /m modifier, which is multi-line mode. The official description is "Treat the string as multiple lines ", this pattern has been included in the regular rules of

JavaScript

since ancient times. The "multiple lines" here means: ^ and $ metacharacters will not match the positions before and after the newline characters in the middle of a string by default, that is It is believed that the string will always have only one line, and it can be matched after turning on the multi-line mode. In other words, single-line mode and multi-line mode are for different metacharacters. People who are new to regular expressions will be confused by the two seemingly corresponding "single-line mode" and "multi-line mode". concept, but in fact, it is confusing with unrelated terms.

Later, the author of Ruby may have felt that the regular term "single-line mode" was not used well, so he called the pattern of "." matching newlines "multi-line mode", that is, let . * and other regular expressions can match multiple lines, so it makes perfect sense. The modifier also uses /m (Ruby will enable the "multiline mode" in Perl by default, so /m is not occupied). This is really To add insult to injury, it’s even more chaotic.

Later, the Python author may also feel that the term "single-line mode" should be avoided, so he gave a new name "dotall", which means that dot can match all characters. It is a good name. , and later Java also used this name.

上面回顾了一下历史，解释了下单行模式的由来以及说明了下单行模式这个名字起得不好。V8 最近刚刚实现了一个 stage 3 的 ES 提案 https://github.com/mathiasbynens/es-regexp-dotall-flag，这个提案为 JavaScript 的正则引入了 /s 修饰符和 dotAll 属性，dotAll 属性是学了 Python 和 Java，/s 修饰符是继承了 Perl 的，这里也没必要发明一个新的修饰符比如 /d，只会让事情更复杂。/s 在 JavaScript 的具体效果是让 “.” 能匹配以前不能匹配的四个行终止符：\n（换行）、\r（回车）、\u2028（行分隔符）、\u2029（段落分隔符）：

/foo/s.dotAll // true
/^.{4}$/s.test("\n\r\u2028\u2029") // true

其实就是个很简单的东西，但可能一些没有接触过 JavaScript 以外的正则的同学到时候学到这个新的模式后会产生困惑，这里再澄清一下：多行模式控制的是 ^ 和 $ 的表现，单行模式控制的是 “.” 的表现，两者没有直接关系。

然而当初引入单行模式和多行模式这两个易混淆概念的 Perl 语言，已经在 Perl 6 中完全删除了这两个模式：“.” 号默认就匹配换行符，\N 可以匹配换行符除外的任意字符；^ 和 $ 始终匹配字符串的首尾，而新引入了 ^^ 和 $$ 两个元字符来匹配行的首尾。

过去我们常用的单行模式的替代品 [^] 或者 [\s\S] 也不是完全没有用了，比如在一些使用 JavaScript 正则的编辑器里（VS Code、Atom），不太可能给你提供开启单行模式的界面。不过说起编辑器里的正则功能，用 JavaScript 实现的编辑器的正则功能还是太弱了，比如不能在正则自身内部开启某些模式，比如要是在 Sublime（使用 Python 正则）里的话，在正则内部使用 (?s) 就能开启 dotall 模式，比如可以用 (?s)/\*.+?\*/ 匹配到所有的多行注释。

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