How to parse a PURL

How to parse a PURL string into its components

Parsing a PURL ASCII string into its components works from right to left, from subpath to type.

Note: some extra type-specific normalizations are required.

To parse a PURL string in its components:

• Split the PURL string once from right on '#'

  • The left side is the remainder
  • Split the right side on '/'
  • Percent-decode each segment
  • UTF-8-decode each segment if needed in your programming language
  • Discard any segment that is empty, or equal to '.' or '..'
  • Report an error if any segment contains a slash '/'
  • This list of path segments is the subpath
  • You may escape these path segments if needed by your environment (operating system, file system, programming language, shell, etc)
  • You may join these path segments with the path delimiter of your environment (operating system, file system, etc)

• Split the remainder once from right on '?'

  • The left side is the remainder

  • The right side is the qualifiers string

  • Split the qualifiers on '&'. Each part is a key=value pair

  • For each pair, split the key=value once from left on '=':

    • The key is the lowercase left side
    • The value is the percent-decoded right side
    • UTF-8-decode the value if needed in your programming language
    • Discard any key/value pairs where the value is empty
    • If the key is 'checksum', split the value on ',' to create a list of checksums

  • This list of key/value is the qualifiers object

• Split the remainder once from left on ':'

  • The left side lowercased is the scheme
  • The right side is the remainder

• Strip all leading '/' characters (e.g., '/', '//', '///' and so on) from the remainder

  • Split this once from left on '/'
  • The left side lowercased is the type
  • The right side is the remainder

• Split the remainder once from right on '@'

  • The left side is the remainder
  • Percent-decode the right side. This is the version.
  • UTF-8-decode the version if needed in your programming language
  • This is the version

• Strip all trailing '/' characters (e.g., '/', '//', '///' and so on) from the remainder

  • Split this once from right on '/'
  • The left side is the remainder
  • Percent-decode the right side. This is the name
  • UTF-8-decode this name if needed in your programming language
  • Apply type-specific normalization to the name if needed
  • This is the name

• Split the remainder on '/'

  • Discard any empty segment from that split
  • Percent-decode each segment
  • UTF-8-decode each segment if needed in your programming language
  • Apply type-specific normalization to each segment if needed
  • Join segments back with a '/'
  • This is the namespace