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TEI-L  May 2018

TEI-L May 2018

Subject:

Re: Question on StackExchange about adding custom attribute to TEI vocabulary:

From:

"C. M. Sperberg-McQueen" <[log in to unmask]>

Reply-To:

C. M. Sperberg-McQueen

Date:

Sat, 5 May 2018 09:06:18 -0600

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (137 lines)

> On May 4, 2018, at 9:47 PM, Elisa Beshero-Bondar <[log in to unmask]> wrote:
> 
> Hi Piotr— I hate to open Pandora’s box here, but can you make a concise summary of Michael’s explanation about the string “ʿmq” and why it’s okay? You’ve piqued my curiosity here! 

I’ll try to save Piotr the trouble.

The short version is that Unicode character U+02BE MODIFIER LETTER
RIGHT HALF RING (the first character in the string "ʿmq") is defined
by the XML specification as a NameStartChar, and thus syntactically
legal at the beginning of an identifier.

Those whose curiosity is now satisfied may stop reading now.

Those interested in more detail (and unafraid of Pandora's Box) may find
the information below suitable reading for a quiet Saturday.

The details of name syntax changed somewhat in the fifth edition of
the specification (issued in 2008), so there are some names which will
be accepted by parsers which conform to XML 1.0 5e but would have been
rejected by older parsers, but the string in question here is not one
of them; it has been a syntactically legal Name in XML since the first
edition of the spec.  The reason for that is, as Vanessa Bigot Juloux
rightly surmises, is that the Unicode database classes it as a Letter;
the first edition of the XML 1.0 specification allows as name-start
characters every character then classified by Unicode as a letter or
as an ideograph.

Beginning with the fifth edition, a somewhat broader range of
characters is allowed, in an attempt to ensure that letters added to
Unicode later can be used in XML names without objection by parsers.
This has the advantage that scripts standardized later (which tend for
perhaps obvious reasons to be scripts for minority languages) are not
disadvantages with respect to being usable for names of XML elements
and attributes; it has the slight disadvantage that when currently
unoccupied code points allowed in XML names are finally defined, there
is no guarantee that they will actually denote letters and not
punctuation special forms of whitespace, or other characters which do
not logically speaking belong inside identifiers.  (I think the
history of XML 1.0 and 1.1 has amply confirmed that the tradeoff made
in the fifth edition is the correct one: the ability to use minority
scripts in names is more important than restricting name characters to
letters, ideographs, numeric digits, and the like.)

The XML specification can readily be consulted on the W3C web site (at
http://www.w3.org/TR/xml); the relevant grammar productions are those
for Name, NameStartChar, and NameChar (5, 4, and 4a in the current
version of the spec).  


[4] NameStartChar ::= ":" | [A-Z] | "_" | [a-z]
    | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF]
    | [#x370-#x37D] | [#x37F-#x1FFF] | [#x200C-#x200D]
    | [#x2070-#x218F] | [#x2C00-#x2FEF]
    | [#x3001-#xD7FF] | [#xF900-#xFDCF]
    | [#xFDF0-#xFFFD] | [#x10000-#xEFFFF]
    
[4a] NameChar ::= NameStartChar | "-" | "." | [0-9]
    | #xB7 | [#x0300-#x036F]
    | [#x203F-#x2040]
    
[5] Name ::= NameStartChar (NameChar)*

U+02BE falls under the definition of NameStartChar by virtue of
the last part of the second line of production 4:  the expression
[#xF8-#x2FF] means (this is explained in the spec’s section on
Notation, you are not expected to be born knowing it, and you
are not required to guess) that any character between U+00F8
and U+02FF is a NameStartChar.

The first and other superseded editions are also still available, 
for those interested; the quickest way to find it is to consult the 
current spec and then click repeatedly on the 'Previous version(s)’ 
links until reaching the one you want (the first edition is 
at https://www.w3.org/TR/1998/REC-xml-19980210).

In order to check a character or string against the spec's definition
of Name, it is of course necessary to know exactly what characters are
in the string; this is made straightforward by the interactive
interfaces offered by some XQuery processors (BaseX, eXist, and
MarkLogic all have some form of this or another; Oxygen also has an
XQuery tool that can be used for the same purpose).  In this case I
copied the string and pasted it into the following query in an XQuery
processor:

    string-to-codepoints("ʿmq")

This produces the result 703 109 113, which tells me that the first
character has the code point 703.  Since Unicode tables use hex
notation, some arithmetic is necessary to translate the decimal 703
into the hexadecimal 2BE, which can be checked against the lists of
character ranges in the spec.

A simpler approach, for those slightly less habituated to consulting
the text of a specification, would be to check whether "ʿmq" can be
coerced into the type xs:NCName, which can be done by issuing the
query

    xs:NCName("ʿmq")

to see whether it returns the NCName ʿmq or raises an error.  Since I
don't use the type coercion functions all that regularly and felt
uncertain whether I was going to get it right, what I actually typed,
eventually, was a slightly more sequence of queries, which compared the
results for this string with those for two simple strings for which I
know what results to expect.  The query

  for $s in ("ʿmq", "abc", "23skiddoo" )
  return <string s="{$s}"
    isname="{xs:NCName($s) instance of xs:NCName}"/>

produces the error message

  [FORG0001] Cannot cast xs:string to xs:NCName: "23skiddoo".

whereas commenting out "23skiddoo" allows the query to succeed:

  for $s in ("ʿmq", "abc" (:, "23skiddoo":) )
  return <string s="{$s}"
    isname="{xs:NCName($s) instance of xs:NCName}"/>

which produces the results

  <string s="ʿmq" isname="true"/>
  <string s="abc" isname="true"/>

I include these details in the hope of encouraging TEI users to
familiarize themselves with tools for interactive evaluation of XPath
and XQuery expressions; they can be very useful.


********************************************
C. M. Sperberg-McQueen
Black Mesa Technologies LLC
[log in to unmask]
http://www.blackmesatech.com
********************************************

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