Updated: Matching line feeds with regular expressions works differently in every language

Changes

Although regular expression syntax is 99% interchangeable between languages, keep this in mind:
- By default, the dot character (`"."`) does **not** match a line feed (newline, line break, `"\n"`) in any language.
- Some languages allow you to modify the behavior of a regular expression by appending a modifier to the pattern expression. E.g. `/foo/i` makes the pattern case-insensitive in many languages. Note however that some of these modifiers may not exist or mean entirely different things in different languages.
- Some languages have a modifier that makes the dot match line feeds.
Here is how to make the dot match line feeds in various languages:
Ruby
-----
You can make the dot match line feeds by using the `/m` modifier. If you come from other languages, do **not** use the `/s` modifier in Ruby. It [changes the RegExp to interpret text as Shift JIS encoded](http://www.tutorialspoint.com/ruby/ruby_regular_expressions.htm) which you probably don't want.
-Javascript
-----------
-There is **no** modifier to make the dot match line feeds. You need to write `[\s\S]`or `(.|\s)` to match any character including line feeds, carriage returns, etc. There's a [TC39 proposal](https://github.com/tc39/proposal-regexp-x-mode) that's stuck in Stage 1.
-While there is a `/m` modifier in Javascript, it only changes the meaning of `^` and `$`.
+## JavaScript
+
+Since ES2018, JavaScript supports the `/s` (dotAll) modifier, which makes `.` match line feeds and other line terminators.
+
+```js
+/foo.bar/s.test("foo\nbar") // true
+```
+
+For compatibility with older JavaScript engines, use `[\s\S]` instead.
+
+JavaScript also has an `/m` (multiline) modifier, but this only changes the meaning of `^` and `$`, not `.`.
+
Perl
----
You can make the dot match line feeds by using the `/s` modifier. Note that there is also `/m`, which does something very different in Perl than in Ruby.