Fix typo in variable names in Regex (#6085)

defmodule Regex do
  @moduledoc ~S"""
  Provides regular expressions for Elixir.

  Regex is based on PCRE (Perl Compatible Regular Expressions) and
  built on top of Erlang's `:re` module. More information can be found
  in the [`:re` module documentation](http://www.erlang.org/doc/man/re.html).

  Regular expressions in Elixir can be created using the sigils
  [`~r`](Kernel.html#sigil_r/2) or [`~R`](Kernel.html#sigil_R/2):

      # A simple regular expressions that matches foo anywhere in the string
      ~r/foo/

      # A regular expression with case insensitive and Unicode options
      ~r/foo/iu

  Regular expressions created via sigils are pre-compiled and stored
  in the `.beam` file. Notice this may be a problem if you are precompiling
  Elixir, see the "Precompilation" section for more information.

  A Regex is represented internally as the `Regex` struct. Therefore,
  `%Regex{}` can be used whenever there is a need to match on them.
  Keep in mind it is not guaranteed two regular expressions from the
  same source are equal, for example:

      ~r/(?<foo>.)(?<bar>.)/ == ~r/(?<foo>.)(?<bar>.)/

  may return `true` or `false` depending on your machine, endianess,
  available optimizations and others. You can, however, retrieve the source
  of a compiled regular expression by accessing the `source` field, and then
  compare those directly:

      ~r/(?<foo>.)(?<bar>.)/.source == ~r/(?<foo>.)(?<bar>.)/.source

  ## Precompilation

  Regular expressions built with sigil are precompiled and stored in `.beam`
  files. This may be a problem if you are precompiling Elixir to run in
  different OTP releases, as OTP releases may update the underlying regular
  expression engine at any time.

  For such reasons, we always recomend precompiling Elixir projects using
  the OTP version meant to run in production. In case cross-compilation is
  really necessary, you can manually invoke `Regex.recompile/1` or `Regex.
  recompile!/1` to perform a runtime version check and recompile the regex
  if necessary.

  ## Modifiers

  The modifiers available when creating a Regex are:

    * `unicode` (u) - enables Unicode specific patterns like `\p` and change
      modifiers like `\w`, `\W`, `\s` and friends to also match on Unicode.
      It expects valid Unicode strings to be given on match

    * `caseless` (i) - adds case insensitivity

    * `dotall` (s) - causes dot to match newlines and also set newline to
      anycrlf; the new line setting can be overridden by setting `(*CR)` or
      `(*LF)` or `(*CRLF)` or `(*ANY)` according to re documentation

    * `multiline` (m) - causes `^` and `$` to mark the beginning and end of
      each line; use `\A` and `\z` to match the end or beginning of the string

    * `extended` (x) - whitespace characters are ignored except when escaped
      and allow `#` to delimit comments

    * `firstline` (f) - forces the unanchored pattern to match before or at the
      first newline, though the matched text may continue over the newline

    * `ungreedy` (U) - inverts the "greediness" of the regexp
      (the previous `r` option is deprecated in favor of `U`)

  The options not available are:

    * `anchored` - not available, use `^` or `\A` instead
    * `dollar_endonly` - not available, use `\z` instead
    * `no_auto_capture` - not available, use `?:` instead
    * `newline` - not available, use `(*CR)` or `(*LF)` or `(*CRLF)` or
      `(*ANYCRLF)` or `(*ANY)` at the beginning of the regexp according to the
      re documentation

  ## Captures

  Many functions in this module handle what to capture in a regex
  match via the `:capture` option. The supported values are:

    * `:all` - all captured subpatterns including the complete matching string
      (this is the default)

    * `:first` - only the first captured subpattern, which is always the
      complete matching part of the string; all explicitly captured subpatterns
      are discarded

    * `:all_but_first`- all but the first matching subpattern, i.e. all
      explicitly captured subpatterns, but not the complete matching part of
      the string

    * `:none` - does not return matching subpatterns at all

    * `:all_names` - captures all names in the Regex

    * `list(binary)` - a list of named captures to capture

  """

  defstruct re_pattern: nil, source: "", opts: "", re_version: ""

  @type t :: %__MODULE__{re_pattern: term, source: binary, opts: binary}

  defmodule CompileError do
    defexception message: "regex could not be compiled"
  end

  @doc """
  Compiles the regular expression.

  The given options can either be a binary with the characters
  representing the same regex options given to the `~r` sigil,
  or a list of options, as expected by the Erlang's `:re` module.

  It returns `{:ok, regex}` in case of success,
  `{:error, reason}` otherwise.

  ## Examples

      iex> Regex.compile("foo")
      {:ok, ~r"foo"}

      iex> Regex.compile("*foo")
      {:error, {'nothing to repeat', 0}}

  """
  @spec compile(binary, binary | [term]) :: {:ok, t} | {:error, any}
  def compile(source, options \\ "") do
    compile(source, options, version())
  end

  defp compile(source, options, version) when is_binary(options) do
    case translate_options(options, []) do
      {:error, rest} ->
        {:error, {:invalid_option, rest}}

      translated_options ->
        compile(source, translated_options, options, version)
    end
  end

  defp compile(source, options, version) when is_list(options) do
    compile(source, options, "", version)
  end

  defp compile(source, opts, doc_opts, version) when is_binary(source) do
    case :re.compile(source, opts) do
      {:ok, re_pattern} ->
        {:ok, %Regex{re_pattern: re_pattern, re_version: version, source: source, opts: doc_opts}}
      error ->
        error
    end
  end

  @doc """
  Compiles the regular expression and raises `Regex.CompileError` in case of errors.
  """
  @spec compile!(binary, binary | [term]) :: t
  def compile!(source, options \\ "") do
    case compile(source, options) do
      {:ok, regex} -> regex
      {:error, {reason, at}} -> raise Regex.CompileError, "#{reason} at position #{at}"
    end
  end

  @doc """
  Recompiles the existing regular expression if necessary.

  This checks the version stored in the regular expression
  and recompiles the regex in case of version mismatch.
  """
  @spec recompile(t) :: t
  def recompile(%Regex{} = regex) do
    version = version()

    # We use Map.get/3 by choice to support old regexes versions.
    case Map.get(regex, :re_version, :error) do
      ^version ->
        {:ok, regex}
      _ ->
        %{source: source, opts: opts} = regex
        compile(source, opts, version)
    end
  end

  @doc """
  Recompiles the existing regular expression and raises `Regex.CompileError` in case of errors.
  """
  @spec recompile!(t) :: t
  def recompile!(regex) do
    case recompile(regex) do
      {:ok, regex} -> regex
      {:error, {reason, at}} -> raise Regex.CompileError, "#{reason} at position #{at}"
    end
  end

  @doc """
  Returns the version of the underlying Regex engine.
  """
  # TODO: No longer check for function_exported? on OTP 20+.
  def version do
    if function_exported?(:re, :version, 0) do
      :re.version()
    else
      "8.33 2013-05-29"
    end
  end

  @doc """
  Returns a boolean indicating whether there was a match or not.

  ## Examples

      iex> Regex.match?(~r/foo/, "foo")
      true

      iex> Regex.match?(~r/foo/, "bar")
      false

  """
  @spec match?(t, String.t) :: boolean
  def match?(%Regex{re_pattern: compiled}, string) when is_binary(string) do
    :re.run(string, compiled, [{:capture, :none}]) == :match
  end

  @doc """
  Returns `true` if the given `term` is a regex.
  Otherwise returns `false`.

  ## Examples

      iex> Regex.regex?(~r/foo/)
      true

      iex> Regex.regex?(0)
      false

  """
  @spec regex?(any) :: boolean
  def regex?(term)
  def regex?(%Regex{}), do: true
  def regex?(_), do: false

  @doc """
  Runs the regular expression against the given string until the first match.
  It returns a list with all captures or `nil` if no match occurred.

  ## Options

    * `:return`  - sets to `:index` to return indexes. Defaults to `:binary`.
    * `:capture` - what to capture in the result. Check the moduledoc for `Regex`
      to see the possible capture values.

  ## Examples

      iex> Regex.run(~r/c(d)/, "abcd")
      ["cd", "d"]

      iex> Regex.run(~r/e/, "abcd")
      nil

      iex> Regex.run(~r/c(d)/, "abcd", return: :index)
      [{2, 2}, {3, 1}]

  """
  @spec run(t, binary, [term]) :: nil | [binary] | [{integer, integer}]
  def run(regex, string, options \\ [])

  def run(%Regex{re_pattern: compiled}, string, options) when is_binary(string) do
    return   = Keyword.get(options, :return, :binary)
    captures = Keyword.get(options, :capture, :all)

    case :re.run(string, compiled, [{:capture, captures, return}]) do
      :nomatch -> nil
      :match   -> []
      {:match, results} -> results
    end
  end

  @doc """
  Returns the given captures as a map or `nil` if no captures are
  found. The option `:return` can be set to `:index` to get indexes
  back.

  ## Examples

      iex> Regex.named_captures(~r/c(?<foo>d)/, "abcd")
      %{"foo" => "d"}

      iex> Regex.named_captures(~r/a(?<foo>b)c(?<bar>d)/, "abcd")
      %{"bar" => "d", "foo" => "b"}

      iex> Regex.named_captures(~r/a(?<foo>b)c(?<bar>d)/, "efgh")
      nil

  """
  @spec named_captures(t, String.t, [term]) :: map | nil
  def named_captures(regex, string, options \\ []) when is_binary(string) do
    names = names(regex)
    options = Keyword.put(options, :capture, names)
    results = run(regex, string, options)
    if results, do: Enum.zip(names, results) |> Enum.into(%{})
  end

  @doc """
  Returns the underlying `re_pattern` in the regular expression.
  """
  @spec re_pattern(t) :: term
  def re_pattern(%Regex{re_pattern: compiled}) do
    compiled
  end

  @doc """
  Returns the regex source as a binary.

  ## Examples

      iex> Regex.source(~r(foo))
      "foo"

  """
  @spec source(t) :: String.t
  def source(%Regex{source: source}) do
    source
  end

  @doc """
  Returns the regex options as a string.

  ## Examples

      iex> Regex.opts(~r(foo)m)
      "m"

  """
  @spec opts(t) :: String.t
  def opts(%Regex{opts: opts}) do
    opts
  end

  @doc """
  Returns a list of names in the regex.

  ## Examples

      iex> Regex.names(~r/(?<foo>bar)/)
      ["foo"]

  """
  @spec names(t) :: [String.t]
  def names(%Regex{re_pattern: re_pattern}) do
    {:namelist, names} = :re.inspect(re_pattern, :namelist)
    names
  end

  @doc ~S"""
  Same as `run/3`, but scans the target several times collecting all
  matches of the regular expression.

  A list of lists is returned, where each entry in the primary list represents a
  match and each entry in the secondary list represents the captured contents.

  ## Options

    * `:return`  - sets to `:index` to return indexes. Defaults to `:binary`.
    * `:capture` - what to capture in the result. Check the moduledoc for `Regex`
      to see the possible capture values.

  ## Examples

      iex> Regex.scan(~r/c(d|e)/, "abcd abce")
      [["cd", "d"], ["ce", "e"]]

      iex> Regex.scan(~r/c(?:d|e)/, "abcd abce")
      [["cd"], ["ce"]]

      iex> Regex.scan(~r/e/, "abcd")
      []

      iex> Regex.scan(~r/\p{Sc}/u, "$, £, and €")
      [["$"], ["£"], ["€"]]

  """
  @spec scan(t, String.t, [term]) :: [[String.t]]
  def scan(regex, string, options \\ [])

  def scan(%Regex{re_pattern: compiled}, string, options) when is_binary(string) do
    return   = Keyword.get(options, :return, :binary)
    captures = Keyword.get(options, :capture, :all)
    options  = [{:capture, captures, return}, :global]

    case :re.run(string, compiled, options) do
      :match -> []
      :nomatch -> []
      {:match, results} -> results
    end
  end

  @doc """
  Splits the given target based on the given pattern and in the given number of
  parts.

  ## Options

    * `:parts` - when specified, splits the string into the given number of
      parts. If not specified, `:parts` defaults to `:infinity`, which will
      split the string into the maximum number of parts possible based on the
      given pattern.

    * `:trim` - when `true`, removes empty strings (`""`) from the result.

    * `:on` - specifies which captures to split the string on, and in what
      order. Defaults to `:first` which means captures inside the regex do not
      affect the splitting process.

    * `:include_captures` - when `true`, includes in the result the matches of
      the regular expression. Defaults to `false`.

  ## Examples

      iex> Regex.split(~r{-}, "a-b-c")
      ["a", "b", "c"]

      iex> Regex.split(~r{-}, "a-b-c", [parts: 2])
      ["a", "b-c"]

      iex> Regex.split(~r{-}, "abc")
      ["abc"]

      iex> Regex.split(~r{}, "abc")
      ["a", "b", "c", ""]

      iex> Regex.split(~r{a(?<second>b)c}, "abc")
      ["", ""]

      iex> Regex.split(~r{a(?<second>b)c}, "abc", on: [:second])
      ["a", "c"]

      iex> Regex.split(~r{(x)}, "Elixir", include_captures: true)
      ["Eli", "x", "ir"]

      iex> Regex.split(~r{a(?<second>b)c}, "abc", on: [:second], include_captures: true)
      ["a", "b", "c"]

  """
  @spec split(t, String.t, [term]) :: [String.t]
  def split(regex, string, options \\ [])

  def split(%Regex{}, "", opts) do
    if Keyword.get(opts, :trim, false) do
      []
    else
      [""]
    end
  end

  def split(%Regex{re_pattern: compiled}, string, opts) when is_binary(string) and is_list(opts) do
    on = Keyword.get(opts, :on, :first)
    case :re.run(string, compiled, [:global, capture: on]) do
      {:match, matches} ->
        do_split(matches, string, 0,
                 parts_to_index(Keyword.get(opts, :parts, :infinity)),
                 Keyword.get(opts, :trim, false),
                 Keyword.get(opts, :include_captures, false))
      :match ->
        [string]
      :nomatch ->
        [string]
    end
  end

  defp parts_to_index(:infinity),                      do: 0
  defp parts_to_index(n) when is_integer(n) and n > 0, do: n

  defp do_split(_, string, offset, _counter, true, _with_captures) when byte_size(string) <= offset,
    do: []

  defp do_split(_, string, offset, 1, _trim, _with_captures),
    do: [binary_part(string, offset, byte_size(string) - offset)]

  defp do_split([], string, offset, _counter, _trim, _with_captures),
    do: [binary_part(string, offset, byte_size(string) - offset)]

  defp do_split([[{pos, _} | h] | t], string, offset, counter, trim, with_captures) when pos - offset < 0,
    do: do_split([h | t], string, offset, counter, trim, with_captures)

  defp do_split([[] | t], string, offset, counter, trim, with_captures),
    do: do_split(t, string, offset, counter, trim, with_captures)

  defp do_split([[{pos, length} | h] | t], string, offset, counter, trim, true) do
    new_offset = pos + length
    keep = pos - offset

    if keep == 0 and length == 0 do
      do_split([h | t], string, new_offset, counter, trim, true)
    else
      <<_::binary-size(offset), part::binary-size(keep), match::binary-size(length), _::binary>> = string

      if keep == 0 and (length == 0 or trim) do
        [match | do_split([h | t], string, new_offset, counter - 1, trim, true)]
      else
        [part, match | do_split([h | t], string, new_offset, counter - 1, trim, true)]
      end
    end
  end

  defp do_split([[{pos, length} | h] | t], string, offset, counter, trim, false) do
    new_offset = pos + length
    keep = pos - offset

    if keep == 0 and (length == 0 or trim) do
      do_split([h | t], string, new_offset, counter, trim, false)
    else
      <<_::binary-size(offset), part::binary-size(keep), _::binary>> = string
      [part | do_split([h | t], string, new_offset, counter - 1, trim, false)]
    end
  end

  @doc ~S"""
  Receives a regex, a binary and a replacement, returns a new
  binary where all matches are replaced by the replacement.

  The replacement can be either a string or a function. The string
  is used as a replacement for every match and it allows specific
  captures to be accessed via `\N` or `\g{N}`, where `N` is the
  capture. In case `\0` is used, the whole match is inserted. Note
  that in regexes the backslash needs to be escaped, hence in practice
  you'll need to use `\\N` and `\\g{N}`.

  When the replacement is a function, the function may have arity
  N where each argument maps to a capture, with the first argument
  being the whole match. If the function expects more arguments
  than captures found, the remaining arguments will receive `""`.

  ## Options

    * `:global` - when `false`, replaces only the first occurrence
      (defaults to `true`)

  ## Examples

      iex> Regex.replace(~r/d/, "abc", "d")
      "abc"

      iex> Regex.replace(~r/b/, "abc", "d")
      "adc"

      iex> Regex.replace(~r/b/, "abc", "[\\0]")
      "a[b]c"

      iex> Regex.replace(~r/a(b|d)c/, "abcadc", "[\\1]")
      "[b][d]"

      iex> Regex.replace(~r/\.(\d)$/, "500.5", ".\\g{1}0")
      "500.50"

      iex> Regex.replace(~r/a(b|d)c/, "abcadc", fn _, x -> "[#{x}]" end)
      "[b][d]"

      iex> Regex.replace(~r/a/, "abcadc", "A", global: false)
      "Abcadc"

  """
  @spec replace(t, String.t, String.t | (... -> String.t), [term]) :: String.t
  def replace(regex, string, replacement, options \\ [])

  def replace(regex, string, replacement, options)
      when is_binary(string) and is_binary(replacement) and is_list(options) do
    do_replace(regex, string, precompile_replacement(replacement), options)
  end

  def replace(regex, string, replacement, options)
      when is_binary(string) and is_function(replacement) and is_list(options)  do
    {:arity, arity} = :erlang.fun_info(replacement, :arity)
    do_replace(regex, string, {replacement, arity}, options)
  end

  defp do_replace(%Regex{re_pattern: compiled}, string, replacement, options) do
    opts = if Keyword.get(options, :global) != false, do: [:global], else: []
    opts = [{:capture, :all, :index} | opts]

    case :re.run(string, compiled, opts) do
      :nomatch ->
        string
      {:match, [mlist | t]} when is_list(mlist) ->
        apply_list(string, replacement, [mlist | t]) |> IO.iodata_to_binary
      {:match, slist} ->
        apply_list(string, replacement, [slist]) |> IO.iodata_to_binary
    end
  end

  defp precompile_replacement(""),
    do: []

  defp precompile_replacement(<<?\\, ?g, ?{, rest::binary>>) when byte_size(rest) > 0 do
    {ns, <<?}, rest::binary>>} = pick_int(rest)
    [List.to_integer(ns) | precompile_replacement(rest)]
  end

  defp precompile_replacement(<<?\\, ?\\, rest::binary>>) do
    [<<?\\>> | precompile_replacement(rest)]
  end

  defp precompile_replacement(<<?\\, x, rest::binary>>) when x in ?0..?9 do
    {ns, rest} = pick_int(rest)
    [List.to_integer([x | ns]) | precompile_replacement(rest)]
  end

  defp precompile_replacement(<<x, rest::binary>>) do
    case precompile_replacement(rest) do
      [head | t] when is_binary(head) ->
        [<<x, head::binary>> | t]
      other ->
        [<<x>> | other]
    end
  end

  defp pick_int(<<x, rest::binary>>) when x in ?0..?9 do
    {found, rest} = pick_int(rest)
    {[x | found], rest}
  end

  defp pick_int(bin) do
    {[], bin}
  end

  defp apply_list(string, replacement, list) do
    apply_list(string, string, 0, replacement, list)
  end

  defp apply_list(_, "", _, _, []) do
    []
  end

  defp apply_list(_, string, _, _, []) do
    string
  end

  defp apply_list(whole, string, pos, replacement, [[{mpos, _} | _] | _] = list) when mpos > pos do
    length = mpos - pos
    <<untouched::binary-size(length), rest::binary>> = string
    [untouched | apply_list(whole, rest, mpos, replacement, list)]
  end

  defp apply_list(whole, string, pos, replacement, [[{pos, length} | _] = head | tail]) do
    <<_::size(length)-binary, rest::binary>> = string
    new_data = apply_replace(whole, replacement, head)
    [new_data | apply_list(whole, rest, pos + length, replacement, tail)]
  end

  defp apply_replace(string, {fun, arity}, indexes) do
    apply(fun, get_indexes(string, indexes, arity))
  end

  defp apply_replace(_, [bin], _) when is_binary(bin) do
    bin
  end

  defp apply_replace(string, repl, indexes) do
    indexes = List.to_tuple(indexes)

    for part <- repl do
      cond do
        is_binary(part) ->
          part
        part >= tuple_size(indexes) ->
          ""
        true ->
          get_index(string, elem(indexes, part))
      end
    end
  end

  defp get_index(_string, {pos, _len}) when pos < 0 do
    ""
  end

  defp get_index(string, {pos, len}) do
    <<_::size(pos)-binary, res::size(len)-binary, _::binary>> = string
    res
  end

  defp get_indexes(_string, _, 0) do
    []
  end

  defp get_indexes(string, [], arity) do
    ["" | get_indexes(string, [], arity - 1)]
  end

  defp get_indexes(string, [h | t], arity) do
    [get_index(string, h) | get_indexes(string, t, arity - 1)]
  end

  @doc ~S"""
  Escapes a string to be literally matched in a regex.

  ## Examples

      iex> Regex.escape(".")
      "\\."

      iex> Regex.escape("\\what if")
      "\\\\what\\ if"

  """
  @spec escape(String.t) :: String.t
  def escape(string) when is_binary(string) do
    string
    |> escape(_length = 0, string)
    |> IO.iodata_to_binary
  end

  @escapable '.^$*+?()[]{}|#-\\\t\n\v\f\r\s'

  defp escape(<<char, rest::binary>>, length, original) when char in @escapable do
    escape_char(rest, length, original, char)
  end

  defp escape(<<_, rest::binary>>, length, original) do
    escape(rest, length + 1, original)
  end

  defp escape(<<>>, _length, original) do
    original
  end

  defp escape_char(<<rest::binary>>, 0, _original, char) do
    [?\\, char | escape(rest, 0, rest)]
  end

  defp escape_char(<<rest::binary>>, length, original, char) do
    [binary_part(original, 0, length), ?\\, char | escape(rest, 0, rest)]
  end

  # Helpers

  @doc false
  # Unescape map function used by Macro.unescape_string.
  def unescape_map(?f), do: ?\f
  def unescape_map(?n), do: ?\n
  def unescape_map(?r), do: ?\r
  def unescape_map(?t), do: ?\t
  def unescape_map(?v), do: ?\v
  def unescape_map(?a), do: ?\a
  def unescape_map(_),  do: false

  # Private Helpers

  defp translate_options(<<?u, t::binary>>, acc), do: translate_options(t, [:unicode, :ucp | acc])
  defp translate_options(<<?i, t::binary>>, acc), do: translate_options(t, [:caseless | acc])
  defp translate_options(<<?x, t::binary>>, acc), do: translate_options(t, [:extended | acc])
  defp translate_options(<<?f, t::binary>>, acc), do: translate_options(t, [:firstline | acc])
  defp translate_options(<<?U, t::binary>>, acc), do: translate_options(t, [:ungreedy | acc])
  defp translate_options(<<?s, t::binary>>, acc), do: translate_options(t, [:dotall, {:newline, :anycrlf} | acc])
  defp translate_options(<<?m, t::binary>>, acc), do: translate_options(t, [:multiline | acc])

  # TODO: Remove on 2.0
  defp translate_options(<<?r, t::binary>>, acc) do
    IO.warn "the /r modifier in regular expressions is deprecated, please use /U instead"
    translate_options(t, [:ungreedy | acc])
  end

  defp translate_options(<<>>, acc), do: acc
  defp translate_options(rest, _acc), do: {:error, rest}
end

defmodule Regex do
  @moduledoc ~S"""
  Provides regular expressions for Elixir.

  Regex is based on PCRE (Perl Compatible Regular Expressions) and
  built on top of Erlang's `:re` module. More information can be found
  in the [`:re` module documentation](http://www.erlang.org/doc/man/re.html).

  Regular expressions in Elixir can be created using the sigils
  [`~r`](Kernel.html#sigil_r/2) or [`~R`](Kernel.html#sigil_R/2):

      # A simple regular expressions that matches foo anywhere in the string
      ~r/foo/

      # A regular expression with case insensitive and Unicode options
      ~r/foo/iu

  Regular expressions created via sigils are pre-compiled and stored
  in the `.beam` file. Notice this may be a problem if you are precompiling
  Elixir, see the "Precompilation" section for more information.

  A Regex is represented internally as the `Regex` struct. Therefore,
  `%Regex{}` can be used whenever there is a need to match on them.
  Keep in mind it is not guaranteed two regular expressions from the
  same source are equal, for example:

      ~r/(?<foo>.)(?<bar>.)/ == ~r/(?<foo>.)(?<bar>.)/

  may return `true` or `false` depending on your machine, endianess,
  available optimizations and others. You can, however, retrieve the source
  of a compiled regular expression by accessing the `source` field, and then
  compare those directly:

      ~r/(?<foo>.)(?<bar>.)/.source == ~r/(?<foo>.)(?<bar>.)/.source

  ## Precompilation

  Regular expressions built with sigil are precompiled and stored in `.beam`
  files. This may be a problem if you are precompiling Elixir to run in
  different OTP releases, as OTP releases may update the underlying regular
  expression engine at any time.

  For such reasons, we always recomend precompiling Elixir projects using
  the OTP version meant to run in production. In case cross-compilation is
  really necessary, you can manually invoke `Regex.recompile/1` or `Regex.
  recompile!/1` to perform a runtime version check and recompile the regex
  if necessary.

  ## Modifiers

  The modifiers available when creating a Regex are:

    * `unicode` (u) - enables Unicode specific patterns like `\p` and change
      modifiers like `\w`, `\W`, `\s` and friends to also match on Unicode.
      It expects valid Unicode strings to be given on match

    * `caseless` (i) - adds case insensitivity

    * `dotall` (s) - causes dot to match newlines and also set newline to
      anycrlf; the new line setting can be overridden by setting `(*CR)` or
      `(*LF)` or `(*CRLF)` or `(*ANY)` according to re documentation

    * `multiline` (m) - causes `^` and `$` to mark the beginning and end of
      each line; use `\A` and `\z` to match the end or beginning of the string

    * `extended` (x) - whitespace characters are ignored except when escaped
      and allow `#` to delimit comments

    * `firstline` (f) - forces the unanchored pattern to match before or at the
      first newline, though the matched text may continue over the newline

    * `ungreedy` (U) - inverts the "greediness" of the regexp
      (the previous `r` option is deprecated in favor of `U`)

  The options not available are:

    * `anchored` - not available, use `^` or `\A` instead
    * `dollar_endonly` - not available, use `\z` instead
    * `no_auto_capture` - not available, use `?:` instead
    * `newline` - not available, use `(*CR)` or `(*LF)` or `(*CRLF)` or
      `(*ANYCRLF)` or `(*ANY)` at the beginning of the regexp according to the
      re documentation

  ## Captures

  Many functions in this module handle what to capture in a regex
  match via the `:capture` option. The supported values are:

    * `:all` - all captured subpatterns including the complete matching string
      (this is the default)

    * `:first` - only the first captured subpattern, which is always the
      complete matching part of the string; all explicitly captured subpatterns
      are discarded

    * `:all_but_first`- all but the first matching subpattern, i.e. all
      explicitly captured subpatterns, but not the complete matching part of
      the string

    * `:none` - does not return matching subpatterns at all

    * `:all_names` - captures all names in the Regex

    * `list(binary)` - a list of named captures to capture

  """

  defstruct re_pattern: nil, source: "", opts: "", re_version: ""

  @type t :: %__MODULE__{re_pattern: term, source: binary, opts: binary}

  defmodule CompileError do
    defexception message: "regex could not be compiled"
  end

  @doc """
  Compiles the regular expression.

  The given options can either be a binary with the characters
  representing the same regex options given to the `~r` sigil,
  or a list of options, as expected by the Erlang's `:re` module.

  It returns `{:ok, regex}` in case of success,
  `{:error, reason}` otherwise.

  ## Examples

      iex> Regex.compile("foo")
      {:ok, ~r"foo"}

      iex> Regex.compile("*foo")
      {:error, {'nothing to repeat', 0}}

  """
  @spec compile(binary, binary | [term]) :: {:ok, t} | {:error, any}
  def compile(source, options \\ "") do
    compile(source, options, version())
  end

  defp compile(source, options, version) when is_binary(options) do
    case translate_options(options, []) do
      {:error, rest} ->
        {:error, {:invalid_option, rest}}

      translated_options ->
        compile(source, translated_options, options, version)
    end
  end

  defp compile(source, options, version) when is_list(options) do
    compile(source, options, "", version)
  end

  defp compile(source, opts, doc_opts, version) when is_binary(source) do
    case :re.compile(source, opts) do
      {:ok, re_pattern} ->
        {:ok, %Regex{re_pattern: re_pattern, re_version: version, source: source, opts: doc_opts}}
      error ->
        error
    end
  end

  @doc """
  Compiles the regular expression and raises `Regex.CompileError` in case of errors.
  """
  @spec compile!(binary, binary | [term]) :: t
  def compile!(source, options \\ "") do
    case compile(source, options) do
      {:ok, regex} -> regex
      {:error, {reason, at}} -> raise Regex.CompileError, "#{reason} at position #{at}"
    end
  end

  @doc """
  Recompiles the existing regular expression if necessary.

  This checks the version stored in the regular expression
  and recompiles the regex in case of version mismatch.
  """
  @spec recompile(t) :: t
  def recompile(%Regex{} = regex) do
    version = version()

    # We use Map.get/3 by choice to support old regexes versions.
    case Map.get(regex, :re_version, :error) do
      ^version ->
        {:ok, regex}
      _ ->
        %{source: source, opts: opts} = regex
        compile(source, opts, version)
    end
  end

  @doc """
  Recompiles the existing regular expression and raises `Regex.CompileError` in case of errors.
  """
  @spec recompile!(t) :: t
  def recompile!(regex) do
    case recompile(regex) do
      {:ok, regex} -> regex
      {:error, {reason, at}} -> raise Regex.CompileError, "#{reason} at position #{at}"
    end
  end

  @doc """
  Returns the version of the underlying Regex engine.
  """
  # TODO: No longer check for function_exported? on OTP 20+.
  def version do
    if function_exported?(:re, :version, 0) do
      :re.version()
    else
      "8.33 2013-05-29"
    end
  end

  @doc """
  Returns a boolean indicating whether there was a match or not.

  ## Examples

      iex> Regex.match?(~r/foo/, "foo")
      true

      iex> Regex.match?(~r/foo/, "bar")
      false

  """
  @spec match?(t, String.t) :: boolean
  def match?(%Regex{re_pattern: compiled}, string) when is_binary(string) do
    :re.run(string, compiled, [{:capture, :none}]) == :match
  end

  @doc """
  Returns `true` if the given `term` is a regex.
  Otherwise returns `false`.

  ## Examples

      iex> Regex.regex?(~r/foo/)
      true

      iex> Regex.regex?(0)
      false

  """
  @spec regex?(any) :: boolean
  def regex?(term)
  def regex?(%Regex{}), do: true
  def regex?(_), do: false

  @doc """
  Runs the regular expression against the given string until the first match.
  It returns a list with all captures or `nil` if no match occurred.

  ## Options

    * `:return`  - sets to `:index` to return indexes. Defaults to `:binary`.
    * `:capture` - what to capture in the result. Check the moduledoc for `Regex`
      to see the possible capture values.

  ## Examples

      iex> Regex.run(~r/c(d)/, "abcd")
      ["cd", "d"]

      iex> Regex.run(~r/e/, "abcd")
      nil

      iex> Regex.run(~r/c(d)/, "abcd", return: :index)
      [{2, 2}, {3, 1}]

  """
  @spec run(t, binary, [term]) :: nil | [binary] | [{integer, integer}]
  def run(regex, string, options \\ [])

  def run(%Regex{re_pattern: compiled}, string, options) when is_binary(string) do
    return   = Keyword.get(options, :return, :binary)
    captures = Keyword.get(options, :capture, :all)

    case :re.run(string, compiled, [{:capture, captures, return}]) do
      :nomatch -> nil
      :match   -> []
      {:match, results} -> results
    end
  end

  @doc """
  Returns the given captures as a map or `nil` if no captures are
  found. The option `:return` can be set to `:index` to get indexes
  back.

  ## Examples

      iex> Regex.named_captures(~r/c(?<foo>d)/, "abcd")
      %{"foo" => "d"}

      iex> Regex.named_captures(~r/a(?<foo>b)c(?<bar>d)/, "abcd")
      %{"bar" => "d", "foo" => "b"}

      iex> Regex.named_captures(~r/a(?<foo>b)c(?<bar>d)/, "efgh")
      nil

  """
  @spec named_captures(t, String.t, [term]) :: map | nil
  def named_captures(regex, string, options \\ []) when is_binary(string) do
    names = names(regex)
    options = Keyword.put(options, :capture, names)
    results = run(regex, string, options)
    if results, do: Enum.zip(names, results) |> Enum.into(%{})
  end

  @doc """
  Returns the underlying `re_pattern` in the regular expression.
  """
  @spec re_pattern(t) :: term
  def re_pattern(%Regex{re_pattern: compiled}) do
    compiled
  end

  @doc """
  Returns the regex source as a binary.

  ## Examples

      iex> Regex.source(~r(foo))
      "foo"

  """
  @spec source(t) :: String.t
  def source(%Regex{source: source}) do
    source
  end

  @doc """
  Returns the regex options as a string.

  ## Examples

      iex> Regex.opts(~r(foo)m)
      "m"

  """
  @spec opts(t) :: String.t
  def opts(%Regex{opts: opts}) do
    opts
  end

  @doc """
  Returns a list of names in the regex.

  ## Examples

      iex> Regex.names(~r/(?<foo>bar)/)
      ["foo"]

  """
  @spec names(t) :: [String.t]
  def names(%Regex{re_pattern: re_pattern}) do
    {:namelist, names} = :re.inspect(re_pattern, :namelist)
    names
  end

  @doc ~S"""
  Same as `run/3`, but scans the target several times collecting all
  matches of the regular expression.

  A list of lists is returned, where each entry in the primary list represents a
  match and each entry in the secondary list represents the captured contents.

  ## Options

    * `:return`  - sets to `:index` to return indexes. Defaults to `:binary`.
    * `:capture` - what to capture in the result. Check the moduledoc for `Regex`
      to see the possible capture values.

  ## Examples

      iex> Regex.scan(~r/c(d|e)/, "abcd abce")
      [["cd", "d"], ["ce", "e"]]

      iex> Regex.scan(~r/c(?:d|e)/, "abcd abce")
      [["cd"], ["ce"]]

      iex> Regex.scan(~r/e/, "abcd")
      []

      iex> Regex.scan(~r/\p{Sc}/u, "$, £, and €")
      [["$"], ["£"], ["€"]]

  """
  @spec scan(t, String.t, [term]) :: [[String.t]]
  def scan(regex, string, options \\ [])

  def scan(%Regex{re_pattern: compiled}, string, options) when is_binary(string) do
    return   = Keyword.get(options, :return, :binary)
    captures = Keyword.get(options, :capture, :all)
    options  = [{:capture, captures, return}, :global]

    case :re.run(string, compiled, options) do
      :match -> []
      :nomatch -> []
      {:match, results} -> results
    end
  end

  @doc """
  Splits the given target based on the given pattern and in the given number of
  parts.

  ## Options

    * `:parts` - when specified, splits the string into the given number of
      parts. If not specified, `:parts` defaults to `:infinity`, which will
      split the string into the maximum number of parts possible based on the
      given pattern.

    * `:trim` - when `true`, removes empty strings (`""`) from the result.

    * `:on` - specifies which captures to split the string on, and in what
      order. Defaults to `:first` which means captures inside the regex do not
      affect the splitting process.

    * `:include_captures` - when `true`, includes in the result the matches of
      the regular expression. Defaults to `false`.

  ## Examples

      iex> Regex.split(~r{-}, "a-b-c")
      ["a", "b", "c"]

      iex> Regex.split(~r{-}, "a-b-c", [parts: 2])
      ["a", "b-c"]

      iex> Regex.split(~r{-}, "abc")
      ["abc"]

      iex> Regex.split(~r{}, "abc")
      ["a", "b", "c", ""]

      iex> Regex.split(~r{a(?<second>b)c}, "abc")
      ["", ""]

      iex> Regex.split(~r{a(?<second>b)c}, "abc", on: [:second])
      ["a", "c"]

      iex> Regex.split(~r{(x)}, "Elixir", include_captures: true)
      ["Eli", "x", "ir"]

      iex> Regex.split(~r{a(?<second>b)c}, "abc", on: [:second], include_captures: true)
      ["a", "b", "c"]

  """
  @spec split(t, String.t, [term]) :: [String.t]
  def split(regex, string, options \\ [])

  def split(%Regex{}, "", opts) do
    if Keyword.get(opts, :trim, false) do
      []
    else
      [""]
    end
  end

  def split(%Regex{re_pattern: compiled}, string, opts) when is_binary(string) and is_list(opts) do
    on = Keyword.get(opts, :on, :first)
    case :re.run(string, compiled, [:global, capture: on]) do
      {:match, matches} ->
        do_split(matches, string, 0,
                 parts_to_index(Keyword.get(opts, :parts, :infinity)),
                 Keyword.get(opts, :trim, false),
                 Keyword.get(opts, :include_captures, false))
      :match ->
        [string]
      :nomatch ->
        [string]
    end
  end

  defp parts_to_index(:infinity),                      do: 0
  defp parts_to_index(n) when is_integer(n) and n > 0, do: n

  defp do_split(_, string, offset, _counter, true, _with_captures) when byte_size(string) <= offset,
    do: []

  defp do_split(_, string, offset, 1, _trim, _with_captures),
    do: [binary_part(string, offset, byte_size(string) - offset)]

  defp do_split([], string, offset, _counter, _trim, _with_captures),
    do: [binary_part(string, offset, byte_size(string) - offset)]

  defp do_split([[{pos, _} | h] | t], string, offset, counter, trim, with_captures) when pos - offset < 0,
    do: do_split([h | t], string, offset, counter, trim, with_captures)

  defp do_split([[] | t], string, offset, counter, trim, with_captures),
    do: do_split(t, string, offset, counter, trim, with_captures)

  defp do_split([[{pos, length} | h] | t], string, offset, counter, trim, true) do
    new_offset = pos + length
    keep = pos - offset

    if keep == 0 and length == 0 do
      do_split([h | t], string, new_offset, counter, trim, true)
    else
      <<_::binary-size(offset), part::binary-size(keep), match::binary-size(length), _::binary>> = string

      if keep == 0 and (length == 0 or trim) do
        [match | do_split([h | t], string, new_offset, counter - 1, trim, true)]
      else
        [part, match | do_split([h | t], string, new_offset, counter - 1, trim, true)]
      end
    end
  end

  defp do_split([[{pos, length} | h] | t], string, offset, counter, trim, false) do
    new_offset = pos + length
    keep = pos - offset

    if keep == 0 and (length == 0 or trim) do
      do_split([h | t], string, new_offset, counter, trim, false)
    else
      <<_::binary-size(offset), part::binary-size(keep), _::binary>> = string
      [part | do_split([h | t], string, new_offset, counter - 1, trim, false)]
    end
  end

  @doc ~S"""
  Receives a regex, a binary and a replacement, returns a new
  binary where all matches are replaced by the replacement.

  The replacement can be either a string or a function. The string
  is used as a replacement for every match and it allows specific
  captures to be accessed via `\N` or `\g{N}`, where `N` is the
  capture. In case `\0` is used, the whole match is inserted. Note
  that in regexes the backslash needs to be escaped, hence in practice
  you'll need to use `\\N` and `\\g{N}`.

  When the replacement is a function, the function may have arity
  N where each argument maps to a capture, with the first argument
  being the whole match. If the function expects more arguments
  than captures found, the remaining arguments will receive `""`.

  ## Options

    * `:global` - when `false`, replaces only the first occurrence
      (defaults to `true`)

  ## Examples

      iex> Regex.replace(~r/d/, "abc", "d")
      "abc"

      iex> Regex.replace(~r/b/, "abc", "d")
      "adc"

      iex> Regex.replace(~r/b/, "abc", "[\\0]")
      "a[b]c"

      iex> Regex.replace(~r/a(b|d)c/, "abcadc", "[\\1]")
      "[b][d]"

      iex> Regex.replace(~r/\.(\d)$/, "500.5", ".\\g{1}0")
      "500.50"

      iex> Regex.replace(~r/a(b|d)c/, "abcadc", fn _, x -> "[#{x}]" end)
      "[b][d]"

      iex> Regex.replace(~r/a/, "abcadc", "A", global: false)
      "Abcadc"

  """
  @spec replace(t, String.t, String.t | (... -> String.t), [term]) :: String.t
  def replace(regex, string, replacement, options \\ [])

  def replace(regex, string, replacement, options)
      when is_binary(string) and is_binary(replacement) and is_list(options) do
    do_replace(regex, string, precompile_replacement(replacement), options)
  end

  def replace(regex, string, replacement, options)
      when is_binary(string) and is_function(replacement) and is_list(options)  do
    {:arity, arity} = :erlang.fun_info(replacement, :arity)
    do_replace(regex, string, {replacement, arity}, options)
  end

  defp do_replace(%Regex{re_pattern: compiled}, string, replacement, options) do
    opts = if Keyword.get(options, :global) != false, do: [:global], else: []
    opts = [{:capture, :all, :index} | opts]

    case :re.run(string, compiled, opts) do
      :nomatch ->
        string
      {:match, [mlist | t]} when is_list(mlist) ->
        apply_list(string, replacement, [mlist | t]) |> IO.iodata_to_binary
      {:match, slist} ->
        apply_list(string, replacement, [slist]) |> IO.iodata_to_binary
    end
  end

  defp precompile_replacement(""),
    do: []

  defp precompile_replacement(<<?\\, ?g, ?{, rest::binary>>) when byte_size(rest) > 0 do
    {ns, <<?}, rest::binary>>} = pick_int(rest)
    [List.to_integer(ns) | precompile_replacement(rest)]
  end

  defp precompile_replacement(<<?\\, ?\\, rest::binary>>) do
    [<<?\\>> | precompile_replacement(rest)]
  end

  defp precompile_replacement(<<?\\, x, rest::binary>>) when x in ?0..?9 do
    {ns, rest} = pick_int(rest)
    [List.to_integer([x | ns]) | precompile_replacement(rest)]
  end

  defp precompile_replacement(<<x, rest::binary>>) do
    case precompile_replacement(rest) do
      [head | t] when is_binary(head) ->
        [<<x, head::binary>> | t]
      other ->
        [<<x>> | other]
    end
  end

  defp pick_int(<<x, rest::binary>>) when x in ?0..?9 do
    {found, rest} = pick_int(rest)
    {[x | found], rest}
  end

  defp pick_int(bin) do
    {[], bin}
  end

  defp apply_list(string, replacement, list) do
    apply_list(string, string, 0, replacement, list)
  end

  defp apply_list(_, "", _, _, []) do
    []
  end

  defp apply_list(_, string, _, _, []) do
    string
  end

  defp apply_list(whole, string, pos, replacement, [[{mpos, _} | _] | _] = list) when mpos > pos do
    length = mpos - pos
    <<untouched::binary-size(length), rest::binary>> = string
    [untouched | apply_list(whole, rest, mpos, replacement, list)]
  end

  defp apply_list(whole, string, pos, replacement, [[{pos, length} | _] = head | tail]) do
    <<_::size(length)-binary, rest::binary>> = string
    new_data = apply_replace(whole, replacement, head)
    [new_data | apply_list(whole, rest, pos + length, replacement, tail)]
  end

  defp apply_replace(string, {fun, arity}, indexes) do
    apply(fun, get_indexes(string, indexes, arity))
  end

  defp apply_replace(_, [bin], _) when is_binary(bin) do
    bin
  end

  defp apply_replace(string, repl, indexes) do
    indexes = List.to_tuple(indexes)

    for part <- repl do
      cond do
        is_binary(part) ->
          part
        part >= tuple_size(indexes) ->
          ""
        true ->
          get_index(string, elem(indexes, part))
      end
    end
  end

  defp get_index(_string, {pos, _len}) when pos < 0 do
    ""
  end

  defp get_index(string, {pos, len}) do
    <<_::size(pos)-binary, res::size(len)-binary, _::binary>> = string
    res
  end

  defp get_indexes(_string, _, 0) do
    []
  end

  defp get_indexes(string, [], arity) do
    ["" | get_indexes(string, [], arity - 1)]
  end

  defp get_indexes(string, [h | t], arity) do
    [get_index(string, h) | get_indexes(string, t, arity - 1)]
  end

  @doc ~S"""
  Escapes a string to be literally matched in a regex.

  ## Examples

      iex> Regex.escape(".")
      "\\."

      iex> Regex.escape("\\what if")
      "\\\\what\\ if"

  """
  @spec escape(String.t) :: String.t
  def escape(string) when is_binary(string) do
    string
    |> escape(_length = 0, string)
    |> IO.iodata_to_binary
  end

  @escapable '.^$*+?()[]{}|#-\\\t\n\v\f\r\s'

  defp escape(<<char, rest::binary>>, lenght, original) when char in @escapable do
    escape_char(rest, lenght, original, char)
  end

  defp escape(<<_, rest::binary>>, lenght, original) do
    escape(rest, lenght + 1, original)
  end

  defp escape(<<>>, _length, original) do
    original
  end

  defp escape_char(<<rest::binary>>, 0, _original, char) do
    [?\\, char | escape(rest, 0, rest)]
  end

  defp escape_char(<<rest::binary>>, lenght, original, char) do
    [binary_part(original, 0, lenght), ?\\, char | escape(rest, 0, rest)]
  end

  # Helpers

  @doc false
  # Unescape map function used by Macro.unescape_string.
  def unescape_map(?f), do: ?\f
  def unescape_map(?n), do: ?\n
  def unescape_map(?r), do: ?\r
  def unescape_map(?t), do: ?\t
  def unescape_map(?v), do: ?\v
  def unescape_map(?a), do: ?\a
  def unescape_map(_),  do: false

  # Private Helpers

  defp translate_options(<<?u, t::binary>>, acc), do: translate_options(t, [:unicode, :ucp | acc])
  defp translate_options(<<?i, t::binary>>, acc), do: translate_options(t, [:caseless | acc])
  defp translate_options(<<?x, t::binary>>, acc), do: translate_options(t, [:extended | acc])
  defp translate_options(<<?f, t::binary>>, acc), do: translate_options(t, [:firstline | acc])
  defp translate_options(<<?U, t::binary>>, acc), do: translate_options(t, [:ungreedy | acc])
  defp translate_options(<<?s, t::binary>>, acc), do: translate_options(t, [:dotall, {:newline, :anycrlf} | acc])
  defp translate_options(<<?m, t::binary>>, acc), do: translate_options(t, [:multiline | acc])

  # TODO: Remove on 2.0
  defp translate_options(<<?r, t::binary>>, acc) do
    IO.warn "the /r modifier in regular expressions is deprecated, please use /U instead"
    translate_options(t, [:ungreedy | acc])
  end

  defp translate_options(<<>>, acc), do: acc
  defp translate_options(rest, _acc), do: {:error, rest}
end