matching module.exs

gistfile1.exs

defmodule Aaa do
  # terminal condition.  If we've gotten through the whole pattern we're done.
  def matches(<<>>, <<>>), do: true

  # If the first character matches and has a question mark we must make an
  # epsilon transition.  We have to try both "universes".  In one "universe"
  # we proceed as if we consumed the optional match, and the other we proceed
  # as if we didn't.  The success of either means the match succeeds.  This
  # means for this branch we don't get TCO =( but this should be relatively rare.
  def matches(<<char, ??, rest1 :: binary>>, string = <<char, rest2 :: binary>>) do
    matches(rest1, rest2) or matches(rest1, string)
  end

  # the case where the match character is optional and both matche is handled 
  # above, so in this case know the first character must mismatch, and we proceed 
  # to the next iteration, ignoring the optional from the match and using the whole
  # rest of the string.
  def matches(<<_, ??, rest1 :: binary>>, string) do
    matches(rest1, string)
  end

  # the case where the match is nonoptional and both match we proceed to the next 
  # iteration, dropping both first characters
  def matches(<<char, rest1 :: binary>>, <<char, rest2 :: binary>>) do
    matches(rest1, rest2)
  end

  # none of the other cases apply so we know it's a mismatch
  def matches(_, _), do: false
end