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 valueif 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 pairs 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.