Named captures in Elixir Regular Expressions

When doing an exercise in Exercism, I learned something cool with regular expressions.

Sometimes you want to fetch parts of the string, so you put brackets in the regular expression.

iex(1)> Regex.run(~r/a(b)c(d)/, "abcde")
["abcd", "b", "d"]

You get a list with all the matches. In this case, you also get a match for the whole matched regex.

That usually is not that useful, so you can skip it with :all_but_first.

iex(2)> Regex.run(~r/a(b)c(d)/, "abcde", capture: :all_but_first)
["b", "d"]

Much better 😊.

Now you can pattern match:

iex(3)> [first, second] = Regex.run(~r/a(b)c(d)/, "abcde", capture: :all_but_first)
["b", "d"]
iex(4)> first
"b"
iex(5)> second
"d"

But later on, you can perhaps do a slight change and include a new match:

iex(6)> [first, second] = Regex.run(~r/(a)(b)c(d)/, "abcde", capture: :all_but_first)
** (MatchError) no match of right hand side value: ["a", "b", "d"]

The way to fix it is by adding a new variable, but:

iex(7)> [first, second, third] = Regex.run(~r/(a)(b)c(d)/, "abcde", capture: :all_but_first)
["a", "b", "d"]
iex(8)> first
"a"
iex(9)> second
"b"
iex(10)> second
"d"

Well, the variables first and second are now capturing another thing.

To fix it you could do this:

iex(11)> [third, first, second] = Regex.run(~r/(a)(b)c(d)/, "abcde", capture: :all_but_first)
["a", "b", "d"]

But everyone will look at you with crazy eyes 🙄🤬

If you need it, give it a name. 😄

You can use something called named captures.

When you open a bracket for a match, add ?<name_of_the_match> with the name of the match.

iex(12)> Regex.named_captures(~r/a(?<first>b)c(?<second>d)/, "abcde")
%{"first" => "b", "second" => "d"}

Pretty neat, right?

For reference: Named Captures with Elixir Regular Expressions