Difference between revisions of "DAP4: DDX Lexical Elements"

From OPeNDAP Documentation
Line 1: Line 1:
At the end of this page is the code for a flex program describing the lexical elements of the DDX. Specifically, it defines
+
This document describes the lexical elements that occur in
* Constants: string, float, integer, char
+
the DAP4 grammar. It is expected that this definition will end
* Identifiers: ID
+
up in the DAP4 specification document. It is also expected that
* References identifiers: IDREF
+
it will be implemented by server and client code using some equivalent
* Whitespace separated lists of IDREF: IDREFS
+
regular expression mechanism.
  
 +
Within the
 +
[http://dl.dropbox.com/u/53929684/xsd.rng Relax-NG (rng) DAP4 grammar],
 +
there are markers for occurrences of
 +
primitive type such as integers, floats, or strings.  The
 +
markers typically look like this when defining an attribute
 +
that can occur in the DAP4 DDX.
 +
<pre>&lt;attribute name=&quot;namespace&quot;&gt;&lt;data type=&quot;string&quot;/&gt;&lt;/attribute&gt;</pre>
 +
The &quot;&lt;data type=&quot;string&quot;/&gt;&quot;
 +
specifies the lexical class for the values that this
 +
attribute can have. In this case, the namespace attribute is
 +
defined to have a String value.  Similar notation is used
 +
for values occurring as text within an xml element.  The
 +
lexical specification later in this document defines the
 +
legal lexical structure for such lexical items.
 +
Specifically, it defines the format of the following lexical
 +
items.
 +
# Constants, namely: string, float, integer, and character.
 +
# Identifiers
  
<font color="green"> I don't understand this proposal. The following sentence and XML snippet imply to me that the flex grammar is specifically designed to parse the values of XML attributes found in the DDX. If that's not the intention then could someone please reword this to more clearly illustrate the intention of this page? Thanks [[User:Ndp|ndp]] 11:10, 27 March 2012 (PDT)</font>
+
The specification is written using the
 +
ISO/IEC 9945-2:2003 Information technology -- Portable Operating System Interface (POSIX) -- Part 2: System Interfaces (see [http://www.iso.org/iso/iso_catalogue/catalogue_ic/catalogue_detail_ics.htm?csnumber=38790]).
 +
This is the extended Posix regular expression specification.
  
Remember that in the DDX, these lexical items will be
+
I have augmented it in the following ways.
enclosed in double quotes, e.g.
+
#Names are assigned to regular expressions using the notation<br>''name = regular-expression''
<Value value="..."/>
+
#Named expressions can be used in subsequent regular expressions by using the notation {name}. Such occurrences are equivalent to textually substituting the expression associated with name for the {name} occurrence: More or less like a macro.
  
 +
===DAP4 Lexical elements===
 +
Notes:
 +
#The definition of {UTF8} is deferred to the next section.
 +
#Comments are indicated using the &quot;//&quot; notation.
 +
#Standard xml escape formats (&amp;xDD) are assumed to be allowed anywhere.
  
''-Dennis Heimbigner''
+
'''Basic character set definitions'''<br>
 +
<pre>
 +
CONTROLS  = [\x00-\x1F] // ASCII control characters
 +
WHITESPACE = [ \r\t\f]+
 +
HEXCHAR    = [0-9a-zA-Z]
 +
// ASCII printable characters
 +
ASCII      = [0-9a-zA-Z !&quot;#$%&amp;'()*+,-./:;&lt;=&gt;?@[\\\]\\^_`|{}~]
 +
</pre>
  
 +
'''Ascii characters that may appear unescaped in Identifiers'''<br>
 +
This is assumed to be basically all ASCII printable characters
 +
except the characters ' ', '.', '/', '&quot;', ' ' ', and '&amp;'.
 +
Occurrences of these characters are assumed to be representable
 +
using the standard xml '&amp;xx;' notation.
 
<pre>
 
<pre>
/* lex specification for tokens for DAP4 DDX */
+
IDASCII    = [0-9a-zA-Z!#$%'()*+,-:;&lt;=&gt;?@[\\\]\\^_`|{}~]
 +
</pre>
  
/* The most correct (validating) version of UTF8 character set
+
'''The numeric classes: integer and float'''<br>
  (Taken from: http://www.w3.org/2005/03/23-lex-U)
+
<pre>
 +
INTEGER    = {INT}|{UINT}|{HEXINT}
 +
INT        = [+-][0-9]+{INTTYPE}?
 +
UINT      = [0-9]+{INTTYPE}?
 +
HEXINT    = {HEXSTRING}{INTTYPE}?
 +
INTTYPE    = ([BbSsLl]|&quot;ll&quot;|&quot;LL&quot;)
 +
HEXSTRING  = (0[xX]{HEXCHAR}+)
 +
</pre>
 +
FLOAT      = ({MANTISSA}{EXPONENT}?)|{NANINF}
 +
EXPONENT  = ([eE][+-]?[0-9]+)
 +
MANTISSA  = [+-]?[0-9]*\.[0-9]*
 +
NANINF    = (-?inf|nan|NaN)
 +
</pre>
 +
'''The Character classes'''<br>
 +
<pre>
 +
STRING    = ([^&quot;\&amp;]|{XMLESCAPE})*
 +
CHARACTER  = ([^'\&amp;]|{XMLESCAPE})
 +
</pre>
  
Note that ASCII and control are not included.
+
Note that the character type only supports ASCII characters because
 +
it can only hold a single 8-bit byte.
  
The lines of the expression cover the UTF8 characters as follows:
+
'''The Identifier class'''<br>
1. non-overlong 2-byte
+
<pre>
2. excluding overlongs
+
ID        = {IDCHAR}+
3. straight 3-byte
+
IDCHAR    = ({IDASCII}|{XMLESCAPE}|{UTF8})
4. excluding surrogates
+
XMLESCAPE  = &amp;x{HEXCHAR}{HEXCHAR};
5. straight 3-byte
+
</pre>
6. planes 1-3
 
7. planes 4-15
 
8. plane 16
 
  
UTF8  ([\xC2-\xDF][\x80-\xBF])                      \
+
Note that the above lexical element classes are not
    | (\xE0[\xA0-\xBF][\x80-\xBF])                  \
+
disjoint. For example, the sequence of characters 1234 can
    | ([\xE1-\xEC][\x80-\xBF][\x80-\xBF])            \
+
be either an identifer,a float, or an integer. So the order
    | (\xED[\x80-\x9F][\x80-\xBF])                  \
+
of testing is assumed to be this.
    | ([\xEE-\xEF][\x80-\xBF][\x80-\xBF])            \
+
#INTEGER
    | (\xF0[\x90-\xBF][\x80-\xBF][\x80-\xBF])        \
+
#FLOAT
    | ([\xF1-\xF3][\x80-\xBF][\x80-\xBF][\x80-\xBF]) \
+
#ID
    | (\xF4[\x80-\x8F][\x80-\xBF][\x80-\xBF])        \
+
#STRING
  
*/
+
===UTF-8 Character Encodings===
 +
We discuss UTF-8 character encoding in the context
 +
of this document: [http://www.w3.org/2005/03/23-lex-U].
  
 +
The most correct (validating) version of UTF8 character set is as follows.
 +
<pre>
 +
UTF8 =  ([\xC2-\xDF][\x80-\xBF])   
 +
      | (\xE0[\xA0-\xBF][\x80-\xBF])   
 +
      | ([\xE1-\xEC][\x80-\xBF][\x80-\xBF])   
 +
      | (\xED[\x80-\x9F][\x80-\xBF])   
 +
      | ([\xEE-\xEF][\x80-\xBF][\x80-\xBF])   
 +
      | (\xF0[\x90-\xBF][\x80-\xBF][\x80-\xBF])   
 +
      | ([\xF1-\xF3][\x80-\xBF][\x80-\xBF][\x80-\xBF])   
 +
      | (\xF4[\x80-\x8F][\x80-\xBF][\x80-\xBF])
 +
</pre>
 +
The lines of the expression cover the UTF8 characters as follows:
 +
# non-overlong 2-byte
 +
# excluding overlongs
 +
# straight 3-byte
 +
# excluding surrogates
 +
# straight 3-byte
 +
# planes 1-3
 +
# planes 4-15
 +
# plane 16
  
/*The most relaxed version of UTF8 (not used)
+
Note that ASCII and control characters are not included.  
UTF8 ([\xC0-\xD6].)|([\xE0-\xEF]..)|([\xF0-\xF7]...)
 
*/
 
  
/*The partially relaxed version of UTF8, and the one used here */
+
The above reference also defines some alternative regular expressions.
UTF8 ([\xC0-\xD6][\x80-\xBF])|([\xE0-\xEF][\x80-\xBF][\x80-\xBF])|([\xF0-\xF7][\x80-\xBF][\x80-\xBF][\x80-\xBF])
 
  
/* ASCII control characters */
+
The most relaxed version of UTF8 is this.
CONTROLS  [\x00-\x1F]
+
<pre>
 +
UTF8 = ([\xC0-\xD6].)
 +
      |([\xE0-\xEF]..)
 +
      |([\xF0-\xF7]...)
 +
</pre>
  
WHITESPACE [ \r\t\f]+
+
The partially relaxed version of UTF8 is this.
 +
<pre>
 +
UTF8    = ([\xC0-\xD6][\x80-\xBF])       
 +
        | ([\xE0-\xEF][\x80-\xBF][\x80-\xBF])       
 +
        | ([\xF0-\xF7][\x80-\xBF][\x80-\xBF][\x80-\xBF])
 +
</pre>
  
HEXCHAR  [0-9a-zA-Z]
+
We deem it acceptable to use this last relaxed expression
 
+
for validating UTF-8 character strings.
/* Generic Escapes */
 
XMLESCAPE  "&x{HEXCHAR}{HEXCHAR};"
 
 
 
/* ASCII printable characters */
 
ASCII    [0-9a-zA-Z !"#$%&'()*+,-./:;<=>?@[\\\]\\^_`|{}~]
 
 
 
/* ASCII Printable Characters minus
 
  ' ','.','/', '"', '&'
 
*/
 
IDASCII  [0-9a-zA-Z!#$%&'()*+,-:;<=>?@[\\\]\\^_`|{}~]
 
 
 
/* Escapes for ' ','.','/', '&', and '"' */
 
IDESCAPES ("&amp;" | "&quot;" | "&x20;" | "&x2E;" | "&x2F;" | "&x26;" | "&x22;")
 
 
 
/* Escapes for '"', '&', and '\\' */
 
STRINGESCAPES ("&amp;" | "&quot;" | "&x26;" | "&x22;" | "&x5C;")
 
 
 
/* Escapes for '\\', '\'' */
 
CHARESCAPES ("&x27;" | "&x5C;")
 
 
 
HEXSTRING      (0[xX]{HEXCHAR}{HEXCHAR}*)
 
 
 
EXPONENT ([eE][+-]?[0-9]+)
 
 
 
MANTISSA [+-]?[0-9]*\.[0-9]*
 
 
 
NANINF  (-?inf|nan|NaN)
 
 
 
INTTYPE  ([BbSsLl]|"ll"|"LL")
 
 
 
INT      [+-][0-9][0-9]*{INTTYPE}?
 
UINT    [0-9][0-9]*{INTTYPE}?
 
HEXINT  {HEXSTRING}{INTTYPE}?
 
 
 
GROUPPATH  [/]?({ID}[/])*{ID}
 
STRUCTPATH ({ID}[.])*{ID}
 
 
 
string  ([^"\\&]|{XMLESCAPE})*
 
 
 
char    ([^'\\&]|{XMLESCAPE})
 
 
 
integer  {INT}|{UINT}|{HEXINT}
 
 
 
float    ({MANTISSA}{EXPONENT}?)|{NANINF}
 
 
 
IDCHAR  ({IDASCII}|{XMLESCAPE}|{UTF8})
 
ID      {IDCHAR}{IDCHAR}*
 
 
 
/* IDREF == path to an object; leads with group path
 
            separated by '/' and then struct path using '.'
 
*/
 
IDREF    {GROUPPATH}{STRUCTPATH}
 
 
 
/* IDREFS is a whitespace separated list of IDREF */
 
IDREFS  {WHITESPACE}?{IDREF}({WHITESPACE}{IDREF})*
 
 
 
%%  /* Order is important */
 
{integer} {}
 
{float}  {}
 
{IDREF}  {}
 
{IDREFS}  {}
 
{ID}      {}
 
{string}  {}
 
 
 
</pre>
 

Revision as of 17:30, 28 March 2012

This document describes the lexical elements that occur in the DAP4 grammar. It is expected that this definition will end up in the DAP4 specification document. It is also expected that it will be implemented by server and client code using some equivalent regular expression mechanism.

Within the Relax-NG (rng) DAP4 grammar, there are markers for occurrences of primitive type such as integers, floats, or strings. The markers typically look like this when defining an attribute that can occur in the DAP4 DDX.

<attribute name="namespace"><data type="string"/></attribute>

The "<data type="string"/>" specifies the lexical class for the values that this attribute can have. In this case, the namespace attribute is defined to have a String value. Similar notation is used for values occurring as text within an xml element. The lexical specification later in this document defines the legal lexical structure for such lexical items. Specifically, it defines the format of the following lexical items.

  1. Constants, namely: string, float, integer, and character.
  2. Identifiers

The specification is written using the ISO/IEC 9945-2:2003 Information technology -- Portable Operating System Interface (POSIX) -- Part 2: System Interfaces (see [1]). This is the extended Posix regular expression specification.

I have augmented it in the following ways.

  1. Names are assigned to regular expressions using the notation
    name = regular-expression
  2. Named expressions can be used in subsequent regular expressions by using the notation {name}. Such occurrences are equivalent to textually substituting the expression associated with name for the {name} occurrence: More or less like a macro.

1 DAP4 Lexical elements

Notes:

  1. The definition of {UTF8} is deferred to the next section.
  2. Comments are indicated using the "//" notation.
  3. Standard xml escape formats (&xDD) are assumed to be allowed anywhere.

Basic character set definitions

CONTROLS   = [\x00-\x1F] // ASCII control characters
WHITESPACE = [ \r\t\f]+
HEXCHAR    = [0-9a-zA-Z]
// ASCII printable characters
ASCII      = [0-9a-zA-Z !"#$%&'()*+,-./:;<=>?@[\\\]\\^_`|{}~]

Ascii characters that may appear unescaped in Identifiers
This is assumed to be basically all ASCII printable characters except the characters ' ', '.', '/', '"', ' ' ', and '&'. Occurrences of these characters are assumed to be representable using the standard xml '&xx;' notation.

IDASCII    = [0-9a-zA-Z!#$%'()*+,-:;<=>?@[\\\]\\^_`|{}~]

The numeric classes: integer and float

INTEGER    = {INT}|{UINT}|{HEXINT}
INT        = [+-][0-9]+{INTTYPE}?
UINT       = [0-9]+{INTTYPE}?
HEXINT     = {HEXSTRING}{INTTYPE}?
INTTYPE    = ([BbSsLl]|"ll"|"LL")
HEXSTRING  = (0[xX]{HEXCHAR}+)

FLOAT = ({MANTISSA}{EXPONENT}?)|{NANINF} EXPONENT = ([eE][+-]?[0-9]+) MANTISSA = [+-]?[0-9]*\.[0-9]* NANINF = (-?inf|nan|NaN)

The Character classes

STRING     = ([^"\&]|{XMLESCAPE})*
CHARACTER  = ([^'\&]|{XMLESCAPE})

Note that the character type only supports ASCII characters because it can only hold a single 8-bit byte.

The Identifier class

ID         = {IDCHAR}+
IDCHAR     = ({IDASCII}|{XMLESCAPE}|{UTF8})
XMLESCAPE  = &x{HEXCHAR}{HEXCHAR};

Note that the above lexical element classes are not disjoint. For example, the sequence of characters 1234 can be either an identifer,a float, or an integer. So the order of testing is assumed to be this.

  1. INTEGER
  2. FLOAT
  3. ID
  4. STRING

2 UTF-8 Character Encodings

We discuss UTF-8 character encoding in the context of this document: [2].

The most correct (validating) version of UTF8 character set is as follows.

UTF8 =   ([\xC2-\xDF][\x80-\xBF])     
       | (\xE0[\xA0-\xBF][\x80-\xBF])     
       | ([\xE1-\xEC][\x80-\xBF][\x80-\xBF])     
       | (\xED[\x80-\x9F][\x80-\xBF])     
       | ([\xEE-\xEF][\x80-\xBF][\x80-\xBF])     
       | (\xF0[\x90-\xBF][\x80-\xBF][\x80-\xBF])     
       | ([\xF1-\xF3][\x80-\xBF][\x80-\xBF][\x80-\xBF])     
       | (\xF4[\x80-\x8F][\x80-\xBF][\x80-\xBF])

The lines of the expression cover the UTF8 characters as follows:

  1. non-overlong 2-byte
  2. excluding overlongs
  3. straight 3-byte
  4. excluding surrogates
  5. straight 3-byte
  6. planes 1-3
  7. planes 4-15
  8. plane 16

Note that ASCII and control characters are not included.

The above reference also defines some alternative regular expressions.

The most relaxed version of UTF8 is this.

UTF8 = ([\xC0-\xD6].)
      |([\xE0-\xEF]..)
      |([\xF0-\xF7]...)

The partially relaxed version of UTF8 is this.

UTF8    = ([\xC0-\xD6][\x80-\xBF])        
        | ([\xE0-\xEF][\x80-\xBF][\x80-\xBF])        
        | ([\xF0-\xF7][\x80-\xBF][\x80-\xBF][\x80-\xBF])

We deem it acceptable to use this last relaxed expression for validating UTF-8 character strings.