lua hostname parser

find_hostname.lua

-- Extract hostname from URI
-- Returns a string containing the hostname
-- Returns nil if the hostname is not found
function _M.find_hostname(subject)
  -- See https://openresty-reference.readthedocs.io/en/latest/Lua_Nginx_API/#ngxrematch for options definition.
  -- In short, j=PCRE JIT compilation, o=compile-once mode, i=case-insensitive mode
  local options = "joi"

  -- See https://tools.ietf.org/html/rfc2396#appendix-B
  -- The regex from RFC2396 was tweaked a little bit to create an optional (0 to 1 times) capturing group around the
  -- two forward slashes (//)? to allow consistent results for URIs that do not have a scheme.
  local captures, err = ngx.re.match(subject, "^(([^:/?#]+):)?((//)?([^/?#]*))?([^?#]*)(\\?([^#]*))?(#(.*))?", options)

  -- captures[0] holds the whole substring being matched
  -- captures[1] holds the first parenthesized sub-pattern's capturing
  -- captures[2] the second, and so on...

  -- For example, http://www.ics.uci.edu/pub/ietf/uri/#Related will give the following results:

  -- scheme    = $2 = captures[2] = http
  -- authority = $5 = captures[5] = www.ics.uci.edu
  -- path      = $6 = captures[6] = /pub/ietf/uri/
  -- query     = $8 = captures[8] = <undefined> (false)
  -- fragment  = $10 = captures[10] = Related

  -- Also, the same url but without the scheme, www.ics.uci.edu/pub/ietf/uri/#Relate, will give the following results:

  -- scheme    = $2 = captures[2] = <undefined> (false)
  -- authority = $5 = captures[5] = www.ics.uci.edu
  -- path      = $6 = captures[6] = /pub/ietf/uri/
  -- query     = $8 = captures[8] = <undefined> (false)
  -- fragment  = $10 = captures[10] = Related

  if captures and captures[5] then
    return captures[5]
  end

  return nil
end