Sigils

Source: Elixir guide, Sigils.

Sigils give the language custom textual literals. They start with ~, then one lowercase letter or one or more uppercase letters, then a delimiter, the content, the closing delimiter, and optional modifiers.

~r/foo/i is sugar for calling sigil_r/2.

Regular expressions

~r builds a PCRE regex. Modifiers go after the last delimiter:

"foo" =~ ~r/foo|bar/    #=> true
"HELLO" =~ ~r/hello/    #=> false
"HELLO" =~ ~r/hello/i   #=> true

Eight delimiters are allowed: / | " ' ( ) [ ] { } < >. Pick one that avoids escaping: ~r(^https?://) reads better than ~r/^https?:\/\//.

Text sigils

Sigil	Builds	Example
~s	a string	~s(has "double" quotes)
~c	a charlist	~c(cat)
~w	a list of words	~w(foo bar bat) gives ["foo", "bar", "bat"]

~w takes c, s or a modifiers for the element type: ~w(foo bar bat)a is [:foo, :bar, :bat].

Lower vs UPPER case

Most lowercase sigils have an uppercase twin. The lowercase one handles escapes (\n, \x26, \u{1F600}, …) and #{} interpolation. The uppercase one takes the text literally.

~S is what you want when writing escapes in docs.

Heredocs (""") work with sigils, so @doc ~S""" avoids double-escaping in documentation.

Calendar sigils

Sigil	Builds
~D[2019-10-31]	%Date{}
~T[23:00:07.0]	%Time{}
~N[2019-10-31 23:00:07]	%NaiveDateTime{}, no timezone ("naive")
~U[2019-10-31 19:59:03Z]	%DateTime{} in UTC

Custom sigils

Define sigil_{char} functions. The content arrives as a string and the modifiers as a charlist:

defmodule MySigils do
  def sigil_i(string, []), do: String.to_integer(string)
  def sigil_i(string, [?n]), do: -String.to_integer(string)
end

import MySigils
~i(13)    #=> 13
~i(42)n   #=> -42

A custom sigil is one lowercase letter, or an uppercase letter followed by uppercase letters and digits. Sigils are often used to embed templating languages or other small languages.