Matching line feeds with regular expressions works differently in every language

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 Show archive.org snapshot which you probably don't want.

JavaScript

Since ES2018, JavaScript supports the /s (dotAll) modifier, which makes . match line feeds and other line terminators.

/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.