Markup encoding/decoding for Common Lisp

There are a lot of markup-esque languages out there (HTML, XML, ...). This is just a simple package that can be used by higher-level markup languages for encoding and decoding strings with tags and character/entity references in them for Common Lisp.

Quickstart

There are only 2 functions exported:

(markup-encode string)
(markup-decode string &key entities)

Encoding with markup-encode will properly convert all standardly encoded characters (e.g. &, <, >, ", and ') into their named entity. All other non-ASCII characters will encode into a character reference.

CL-USER > (markup-encode "<This & That>")
"&lt;This &amp; That&gt;"

Likewise, markup-decode will perform the inverse.

CL-USER > (markup-decode *)
"<This & That>"

Note: encoding and decoding will always return a new string, even if there is nothing to encode or decode.

The markup-decode function also takes an optional entities keyword argument. This allows for passing in of document-specific entities. There are already a plethora of common entities that are tested against when decoding, but (XML for example) some markup languages allow the user to define their own entities. If passed in, document entities take priority.

CL-USER > (markup-decode "• Item 1.a. is &cool;" :entities '(("cool" . "AWESOME!")))
"• Item 1.a. is AWESOME!"

Customizing Entity References

Different markup languages use different predicates for what make up a valid entity reference character. You can override the predicates used with the special variables *markup-entity-start-char-p* and *markup-entity-char-p*.

Note: by default, these two variables default to alpha-char-p.

Simply set these to what you'd like them to be for your markup language and then decode the markup. For example:

(let ((*markup-entity-start-char-p* 'xml-name-char-p)
      (*markup-entity-char-p* 'xml-token-char-p))
  (markup-decode "&my-valid.xml:entity;"))

Preventing Infinite Expansion

Markup languages typically allow for recursive expansion of entities. For example, in XML:

<!DOCTYPE doc [
<!ENTITY a "&b;">
<!ENTITY b "&c;">
<!ENTITY c "Hello, world!">
]>

If you were to use &a; in your document, it would expand to "Hello, world!" appropriately. However, malformed entity references can be used to exploit code. Circular entity references can cause infinite loops, and grossly expanding entities can allow for out of memory crashes.

To help mitigate this, the *markup-entity-level* special variable allows you to cap the maximum level of expansion recursion that can take place. By default this is set to 3 - a good safe value.

That's it!