tDOM manual: schema

## NAME

tdom::schema -
Creates a schema validation command

## SYNOPSIS

    package require tdom

    tdom::schema ?create? cmdName

## DESCRIPTION

Every call of this command creates a new validation command. A validation command has methods to define a schema and is able to validate XML data or to post-validate a tDOM DOM tree (and to some degree other kind of hierarchical data) against this schema.

Also, a validation command may be used as argument to the -validateCmd option of the dom parse and the expat commands to enable validation additionally to what they do otherwise.

The methods of created commands are:

prefixns ?prefixUriList?
  This method controls prefix (or abbreviation) to namespace URI mapping. Wherever a namespace argument is expected in the schema command methods the "prefix" could be used instead of the namespace URI. If the list maps the same prefix to different namespace URIs, the first one wins. If there is no such prefix, the namespace argument is used literally as namespace URI. If the method is called without argument, it returns the current prefixUriList. If the method is called with the empty string, any namespace URI arguments are used literally. This is the default.
defelement name ?namespace? <definition script>
  This method defines the element name (optional in the namespace namespace) in the schema. The definition script is evaluated and defines the content model of the element. If the namespace argument is given, any element or ref references in the definition script not wrapped inside a namespace command are resolved in that namespace. If there is already a element definition for the name/namespace combination, the command raises error.
defelementtype typename name ?namespace? <definition script>
  This method defines the element type typename (optional in the namespace namespace) in the schema. If the element type is used in a definition script with the schema command elementtype, the validation engine expects an element named name (in the namespace namespace, if given) and the content model definition script. Defining element types seems only sensible if you really have elements with the same name and namespace but different content models. The definition script is evaluated and defines the content model of the element. If the namespace argument is given, any element or ref references in the definition script not wrapped inside a namespace command are resolved in that namespace. If there is already an elementtype definition for the name/namespace combination, the command raises error. The document element of any XML to validate cannot be a defelementtype defined element.
defpattern name ?namespace? <definition script>
  This method defines a (maybe complex) content particle with the name (optional in the namespace namespace) in the schema, to be used in other definition scripts with the definition command ref. The definition script is evaluated and defines the content model of the content particle. If the namespace argument is given, any element or ref references in the definition script not wrapped inside a namespace command are resolved in that namespace. If there is already a pattern definition for the name/namespace combination, the command raises error.
deftexttype name <constraint script>
  This method defines a bundle of text constraints that can be referred to by name while defining constraints on text element or attribute values. If there is already a text type definition with this name, the command raises error. A text type must be defined before it can be used in schema definition scripts.
start documentElement ?namespace?
  This method defines the name and namespace of the root element of a tree to validate. If this method is used, the root element must match for validity. If start is not used, any element defined by defelement may be the root of a valid document. The start method may be used several times with varying arguments during the lifetime of a validation command. If the command is called with just the empty string (and no namespace argument), the validation constraint for the root element is removed and any defined element will be valid as root of a tree to validate.
define <definition script>
  This method allows to define several elements or patterns or a whole schema with one call. All schema command methods so far (prefixns, defelement, defelementtype, defpattern, deftexttype and start) are allowed top level in the definition script. The define method itself isn't allowed recursively.
event (start|end|text) ?event specific data?
  This method allows the validation of hierarchical data against the content constraints of the validation command.
  start name ?attributes? ?namespace?
    Checks if the current validation state allows the element name in the namespace to start here. It raises error if not.
  end
    Checks if the current innermost open element may end there in the current state without violation of validation constraints. It raises error if not.
  text text
    Checks if the current validation state allows the given text content. It raises error if not.
validate <XML string> ?objVar?
  Returns true if the <XML string> is valid, or false, otherwise. If validation has failed and the optional objVar argument is given, the variable with that name is set to a validation error message. If the XML string is valid and the optional objVar argument is given, the variable with that name is set to the empty string.
validatefile filename ?objVar?
  Returns true if the content of filename is valid, or false, otherwise. The given file is feeded as binary stream to expat, therefore only US-ASCII, ISO-8859-1, UTF-8 or UTF-16 encoded data will work with this method. If validation has failed and the optional objVar argument is given, the variable with that name is set to a validation error message. If the XML string is valid and the optional objVar argument is given, the variable with that name is set to the empty string.
validatechannel channel ?objVar?
  Returns true if the content read from the Tcl channel channel is valid, or false, otherwise. Since data read out of a Tcl channel is UTF-8 encoded, any misleading encoding declaration at the beginning of the data will lead to errors. If the validation fails and the optional objVar argument is given, the variable with that name is set to a validation error message. If the XML string is valid and the optional objVar argument is given, the variable with that name is set to the empty string.
domvalidate domNode ?objVar?
  Returns true if the first argument is a valid tree, or false, otherwise. If validation has failed and the optional objVar argument is given, the variable with that name is set to a validation error message. If the dom tree is valid and the optional objVar argument is given, the variable with that name is set to the empty string.
reportcmd ?cmd?
  This method expects the name of a Tcl command to be called in case of validation error. The command will be called with two arguments appended: the schema command which raises the validation error, and a validation error code.

  The possible error codes are:

  MISSING_ELEMENT
  MISSING_TEXT
  UNEXPECTED_ELEMENT
  UNEXPECTED_ROOT_ELEMENT
  UNEXPECTED_TEXT
  UNKNOWN_ROOT_ELEMENT
  UNKNOWN_ATTRIBUTE
  MISSING_ATTRIBUTE
  INVALID_ATTRIBUTE_VALUE
  DOM_KEYCONSTRAINT
  DOM_XPATH_BOOLEAN
  INVALID_KEYREF
  INVALID_VALUE
  UNKOWN_GLOBAL_ID
  UNKOWN_ID

  For more detailed information see section Recovering.

delete
  This method deletes the validation command.
info ?args?
  This method bundles methods to query the state of and details about the schema command.
  validationstate
    This method returns the state of the validation command with respect to validation state. The possible return values and their meanings are:
    READY
      The validation command is ready to start validation
    VALIDATING
      The validation command is in the process of validating input.
    FINISHED
      The validation has finished, no further events are expected.
  vstate
    This method is a shorter alias for validationstate; see there.
  line
    If the schema command is currently validating, this method returns the line part of the parsing position information, and the empty string in all other cases. If the schema command is currently post-validating a DOM tree, there may be no position information stored at some or all nodes. The empty string is returned in these cases.
  column
    If the schema command is currently validating this method returns the column part of the parsing position information, and the empty string in all other cases. If the schema command is currently post-validating a DOM tree, there may be no position information stored at some or all nodes. The empty string is returned in these cases.
  domNode
    If the schema command isn't currently post-validating a DOM tree this method returns the empty string. Otherwise, if the schema command waits for the reportcmd script to finish while recovering from a validation error it returns the node on which the validation engine is currently looking at in case the node is an ELEMENT_NODE or, if not, its parent node. It is recommended that you do not use this method. Or at least leave the DOM tree alone, use it read-only.
  nrForwardDefinitions
    Returns how many elements, element types and ref patterns are referenced that aren't defined so far (summed together).
  definedElements
    Returns in no particular order the defined elements in the grammar as list. If an element is namespaced, its list entry will be itself a list with two elements, with the name as first and the namespace as second element.
  definedElementtypes
    Returns in no particular order the defined element types in the grammar as list. If an element type is namespaced, its list entry will be itself a list with two elements, with the name as first and the namespace as second element.
  definedPatterns
    Returns in no particular order the defined named pattern in the grammar as list. If a named pattern is namespaced, its list entry will be itself a list with two elements, with the name as first and the namespace as second element.
  expected
    Returns in no particular order all possible next events (since the last successful event match, if there was one) as a list. If an element is namespaced its list entry will be itself a list with two elements, with the name as first and the namespace as second element. If text is a possible next event, the list entry will be a two elements list, with #text as first element and the empty string as second. If an any element constraint is possible. the list entry will be a two elements list, with <any> as first element and the empty string as second. If an any element in a certain namespace constraint is possible, the list entry will be a two elements list, with <any> as first element and the namespace as second. If element end is a possible event, the list entry will be a two elements list with <elementend> as first element and the empty string as second element.
  definition name ?namespace?
    Returns the code that defines the given element. The command raises error if there is no definition of that element.
  typedefinition name ?namespace?
    Returns the code that defines the given element type definition. The command raises error if there is no definition of that element.
  patterndefinition name ?namespace?
    Returns the code that defines the given pattern definition. The command raises error if there is no definition of a pattern with that name and, if given, namespace.
  vaction ?name|namespace|text?

    This method returns useful information only if the schema command waits for the reportcmd script to finish while recovering from a validation error. Otherwise it returns NONE.

    If the command is called without the optional argument the possible return values and their meanings are:

    NONE
      The schema command currently does not recover from a validation event.
    MATCH_ELEMENT_START
      Element start event, which includes looking for missing or unknown attributes.
    MATCH_ELEMENT_END
      Element end event.
    MATCH_TEXT
      Validating text between tags.
    MATCH_ATTRIBUTE_TEXT
      Attribute text value constraint check
    MATCH_GLOBAL
      Checking global IDs
    MATCH_DOM_KEYCONSTRAINT
      Checking domunique constraint
    MATCH_DOM_XPATH_BOOLEAN
      Checking domxpathboolean constant

    If called with one of the possible optional arguments, the command returns detail information depending on current action.

    name
      Returns the name of the element that has to match in case of MATCH_ELEMENT_START. Returns the name of the closed element in case of MATCH_ELEMENT_END. Returns the name of the attribute in case of MATCH_ATTRIBUTE_TEXT. Returns the name of the parent element in case of MATCH_TEXT.
    namespace
      Returns the namespace of the element that has to match in case of MATCH_ELEMENT_START. Returns the namespace of the closed element in case of MATCH_ELEMENT_END. Returns the namespace of the attribute in case of MATCH_ATTRIBUTE_TEXT. Returns the namespace of the parent element in case of MATCH_TEXT.
    text
      Returns the text to match in case of MATCH_TEXT. Returns the value of the attribute in case of MATCH_ATTRIBUTE_TEXT.
  stack top|inside|associated
    In Tcl scripts evaluated by validation this method provides information about the current validation stack. Called outside this context the method returns the empty string.
    top
      Returns the element whose content is currently checked (the open element tag at this moment).
    inside
      Returns all currently open elements as a list.
    associated
      Returns the data associated with the current top most stack content particle or the empty string if there isn't any.
reset
  This method resets the validation command into state READY (while preserving the defined grammar).

## Schema definition scripts

Schema definition scripts are ordinary Tcl scripts evaluated in the namespace tdom::schema. The schema definition commands listed below in this Tcl namespace allow the definition of a wide variety of document structures. Every schema definition command establishes a validation constraint on the content which has to match or must be optional to qualify the content as valid. It is a validation error if there is additional (not matched) content. White-space-only text (in the XML sense of white space) between any different tags is ignored, with the exception of text only elements (for which even white-space-only text will be considered as significant content).

The schema definition commands are:

element name ?quant? ?<definition script>?
  If the optional argument definition script is not given this command refers to the element defined with defelement with the name name in the current context namespace. If the defelement script argument is given, the validation constraint expects an element with the name name in the current namespace with content "locally" defined by the definition script. Forward references to so far not defined elements or patterns or other local definitions of the same name inside the definition script are allowed. If a forward referenced element is not defined until validation, only an empty element with name name and namespace namespace and no attributes matches.
elementtype name ?quant?
  This command refers to the element defined with defelementtype with the type name name in the current context namespace. Forward references to so far not defined element types or recursive references are allowed. If a forward referenced element type is not defined until validation any empty element without attributes will be accepted.
ref name ?quant?
  This command refers to the content particle defined with defpattern with the name name in the current context namespace. Forward references to a so far not defined pattern and recursive references are allowed. If a forward referenced pattern is not defined until validation no content whatsoever is expected ("empty match").
group ?quant? <definition script>
  This method allows to group a sequence of content particles defined by the definition script>, which have to match in this sequence order.
choice ?quant? <definition script>
  This schema constraint matches if one of the top level content particles defined by the definition script> matches. If one of this top level content particle is optional this constraint matches the "empty match".
interleave ?quant? <definition script>
  This schema constraint matches after every of the required top level content particles defined by the definition script> have matched (and, optional, some or all other) in any arbitrary order.
mixed ?quant? <definition script>
  This schema constraint matches for any text (including the empty one) and every top level content particle defined by the definition script> with default quantifier *.
text ?<constraint script>|"type" typename?
  Without the optional constraint script this validation constraint matches every string (including the empty one). With constraint script or with a given text type argument a text matching this script or the text type is expected.
any ?namespace? ?quant?
  The any command matches every element (in the namespace namespace, if that is given) (with whatever attributes) or subtree, no matter if known within the schema or not. Please note that in case of no namespace argument is given that means that the quantifier * and + will eat up any elements until the enclosing element ends. If you really have a namespace that looks like a valid tDOM schema quantifier you will have to spell out always both arguments.
attribute name ?quant? (?<constraint script>|"type" typename?)
  The attribute command defines an attribute (in no namespace) to the enclosing element. The first definition of name inside an element definition wins; later definitions of the same name are silently ignored. After the name argument there may be one of the quantifiers ? or !. If there is, it will be used. Otherwise the attribute will be required (must be present in the XML source). If there is one argument more this argument is evaluated as constraint script, defining the value constraints of the attribute. Otherwise, if there are two more arguments and the first of them is the bare-word "type" the following argument is used as a text type name. This command is only allowed at top level in the definition script of an defelement/element script.
nsattribute name namespace ?quant? (?<constraint script>|"type" typename?)
  This command does the same as the command attribute, for the attribute name in the namespace namespace.
namespace URI <definition script>
  Evaluates the definition script with context namespace URI. Every element, element type or ref command name will be looked up in the namespace URI, and local defined elements will be in that namespace. An empty string as URI means no namespace.
tcl tclcmd ?arg arg ...?
  Evaluates the Tcl script tclcmd arg arg ... . This validation command is only allowed in strict sequential context (not in choice, mixed and interleave). If the return code is something else than TCL_OK, this is an error (which is not catched and reported by reportcmd).
self
  Returns the schema command.
associate data
  This command is only allowed top-level inside definition scripts of the element, elementtype, pattern or interleave content particles. Associates the data given as argument with the currently defined content particle and may be requested in scripts evaluated while validating the content of that particle with the schema command method call info stack associated.
domunique selector fieldlist ?name? ?"IGNORE_EMPTY_FIELD_SET"|("EMPTY_FIELD_SET_VALUE" emptyFieldSetValue)?
  If not postvalidating a DOM tree with domvalidate this constraint always matches. If postvalidating this constraint resembles the xsd key/keyref mechanism. The selector argument may be any valid XPath expression (without the xsd limits). Several domunique commands within one element definition are allowed. They are checked in definition order. The argument name is available in the recovering script per info vaction name. If the fieldlist does not select something for a node of the result set of the selector the key value will be the empty string by default. If the arguments EMPTY_FIELD_SET_VALUE <value> are given an empty node set will have the key value value. If instead the flag IGNORE_EMPTY_FIELD_SET flag is given an empty node set result will not have any key value.
domxpathboolean XPath_expr ?name?

  If not postvalidating a DOM tree with domvalidate this constraint always matches. If postvalidating the XPath_expr argument is evaluated (with the node matching the schema parent of the domxpathboolean command as context node). The constraint maches if the result of this XPath expression, converted to boolean by XPath rules, is true. Several domxpathboolean commands within one element definition are allowed. They are checked in definition order.

  This enables checks depending on more than one element. Consider

      tdom::schema s
      s define {
          defelement doc {
              element a ! text
              element b ! text
              element c ! text
              domxpathboolean "a * b * c >= 20000" volume
              domxpathboolean "a > b and b > c" sequence
          }
      }

prefixns ?prefixUriList?
  This defines a prefix to namespace URI mapping exactly as a schemacmd prefixns would. It is meant as top-level command of a schemacmd define script. This command is not allowed nested in another definition script command and will raise error, if you call it there.
defelement name ?namespace? <definition script>
  This defines an element exactly as a schemacmd defelement call would. It is meant as top-level command of a schemacmd define script. This command is not allowed nested in another definition script command and will raise error, if you call it there.
defelementtype name ?namespace? <definition script>
  This defines an elementtype exactly as a schemacmd defelementtype call would. It is meant as top-level command of a schemacmd define script. This command is not allowed nested in another definition script command and will raise error, if you call it there.
defpattern name ?namespace? <definition script>
  This defines a named pattern exactly as a schemacmd defpattern call would. It is meant as top-level command of a schemacmd define script. This command is not allowed nested in another definition script command and will raise error, if you call it there.
deftexttype name <constraint script>
  This defines a named bundle of text constraints exactly as a schemacmd deftexttype call would. It is meant as top-level command of a schemacmd define script. This command is not allowed nested in another definition script command and will raise error, if you call it there.
start name ?namespace?
  This command works exactly as a schemacmd start call would. It is meant as top-level command of a schemacmd define script. This command is not allowed nested in another definition script command and will raise error, if you call it there.

## Quantity specifier

Several schema definition commands expect a quantifier as one of their arguments which determines how often the content particle specified by the command is expected. The valid values for a quant argument are:

!
  The content particle has to occur exactly once in valid documents.
?
  The content particle may not occur more than once in valid documents - the particle is optional.
*
  The content particle may occur zero or more times in a row in valid documents.
+
  The content particle may occur one or more times in a row in valid documents.
n
  The content particle must occur n times in a row in valid documents. The quantifier must be an integer greater zero.
{n m}
  The content particle must occur at least n and at most m times in a row in valid documents. The quantifier must be a Tcl list with two elements. Both elements must be integers, with n >= 0 and n < m.

If an optional quantifier is not given, it defaults to * in case of the mixed command and to ! for all other commands.

## Text constraint scripts

Text (parsed character data, as XML calls it) sometimes has to be of a certain kind or comply with certain rules to be valid. The text constraint script arguments to text, attribute, nsattribute and deftexttype commands are evaluated in the Tcl namespace tdom::schema::text namespace and allow the ensuing text constraint commands to check text for certain properties. The commands are defined in the Tcl namespace tdom::schema::text. They raise error in case they are called outside of a text constraint script.

A few of the ensuing text type commands are exposed as general Tcl commands. They are defined in the namespace tdom::type and are called as documented below with the text to check appended to the argument list. They return a logical value. Please note that the commands may not accept starting or ending white space. If a command is available in the tdom::type namespace is recorded in its documentation.

### The tcl text constraint command

The tcl text constraint command dispatches the check to an arbitrary Tcl command, thus enable any programmable decision rules.

tcl tclcmd ?arg arg ...?
  Evaluates the Tcl script tclcmd arg arg ... and the text to validate appended to the argument list. The return value of the Tcl command is interpreted as a boolean.

### Basic XML types

name
  This text constraint matches if the text value matches the XML name production https://www.w3.org/TR/xml/#NT-Name. This means that the text value must start with a letter, underscore (_), or colon (:), and may contain only letters, digits, underscores (_), colons (:), hyphens (-), and periods (.).
ncname
  This text constraint matches if the text value matches the XML ncname production https://www.w3.org/TR/xml-names/#NT-NCName. This means that the text value must start with a letter or underscore (_), and may contain only letters, digits, underscores (_), hyphens (-), and periods (.) (The only difference to the name constraint is that colons are not permitted.)
qname
  This text constraint matches if the text value matches the XML qname production https://www.w3.org/TR/xml-names/#NT-QName. This means that the text value is either a ncname or two ncnames joined by a colon (:).
nmtoken
  This text constraint matches if the text value matches the XML nmtoken production https://www.w3.org/TR/xml/#NT-Nmtoken
nmtokens
  This text constraint matches if the text value matches the XML nmtokens production https://www.w3.org/TR/xml/#NT-Nmtokens

### Basic type tests

integer ?(xsd|tcl)?
  This text constraint matches if the text value could be parsed as an integer. If the optional argument to the command is tcl, everything that returns TCL_OK if feeded into Tcl_GetInt() matches. If the optional argument to the command is xsd, the constraint matches if the value is a valid xsd:integer. Without argument xsd is the default.
negativeInteger ?(xsd|tcl)?
  This text constraint matches the same text values as the integer text constraint (see there), with the additional constraint, that the value must be < zero.
nonNegativeInteger ?(xsd|tcl)?
  This text constraint matches the same text values as the integer text constraint (see there), with the additional constraint, that the value must be >= zero.
nonPositiveInteger ?(xsd|tcl)?
  This text constraint matches the same text values as the integer text constraint (see there), with the additional constraint, that the value must be <= zero.
positiveInteger ?(xsd|tcl)?
  This text constraint matches the same text values as the integer text constraint (see there), with the additional constraint, that the value must be > zero.
number ?(xsd|tcl)?
  This text constraint matches if the text value could be parsed as a number. If the optional argument to the command is tcl, everything that returns TCL_OK if feeded into Tcl_GetDouble() matches. If the optional argument to the command is xsd, the constraint matches if the value is a valid xsd:decimal. Without argument xsd is the default.
boolean ?(xsd|tcl)?
  This text constraint matches if the text value could be parsed as a boolean. If the optional argument to the command is tcl, everything that returns TCL_OK if feeded into Tcl_GetBoolean() matches. If the optional argument to the command is xsd, the constraint matches if the value is a valid xsd:boolean. Without argument xsd is the default.
date
  This text constraint matches if the text value is a xsd:date, which is basically like an ISO 8601 date of the form YYYY-MM-DD, with optional time zone part (either the letter Z or plus (+) or minus (-) followed by hh:mm and with maximum allowed positive or negative time zone 14:00). It follows the date rules of the Gregorian calendar for all dates. A preceding minus sign for bce dates is allowed. There is no year 0. The year may have more than 4 digits, but only if needed (no extra leading zeros). This is available as common Tcl command tdom::type::date.
time
  This text constraint matches if the text value is a xsd:time, which is basically like an ISO 8601 time of the form hh:mm:ss with optional time zone part. The time zone part follow the rules of the date command; see there. All three parts of the time value (hours, minutes, seconds) must be spelled out with 2 digits. Additional fractional seconds (with a point ('.') as separator) are allowed, but not just a dangling point. The time value 24:00:00 (without fractional part) is allowed. This is available as common Tcl command tdom::type::time.
dateTime
  This text constraint matches if the text value is a xsd:dateTime, which is basically like an ISO 8601 date time of the form YYYY-MM-DDThh:mm:ss with optional time zone part. The date and time zone parts follows the rules of the date and time command; see there. The time part (including the signaling 'T' character) is mandatory. This is available as common Tcl command tdom::type::dateTime.
duration
  This text constraint matches if the text value is a xsd:duration, which is basically like an ISO 8601 duration of the form PnYnMnDTnHnMnS. All parts other than the starting P and - if one of H, M or S is given - T are optional. In case the following sign letter is S, n may be a decimal (with at least one digit before and after the dot), otherwise it must be a (positive) integer. This is available as common Tcl command tdom::type::duration.
base64
  This text constraint matches if text is valid according to RFC 4648.
hexBinary
  This text constraint matches if text is a sequence of binary octets in hexadecimal encoding, where each binary octet is a two-character hexadecimal number. Lowercase and uppercase letters A through F are permitted.
unsignedByte
  This text constraint matches if the text value is a xsd:unsignedByte. This is an integer between 0 and 255, both included, optionally preceded by a + sign and leading zeros.
unsignedShort
  This text constraint matches if the text value is a xsd:unsignedShort. This is an integer between 0 and 65535, both included, optionally preceded by a + sign and leading zeros.
unsignedInt
  This text constraint matches if the text value is a xsd:unsignedInt. This is an integer between 0 and 4294967295, both included, optionally preceded by a + sign and leading zeros.
unsignedLong
  This text constraint matches if the text value is a xsd:unsignedLong. This is an integer between 0 and 18446744073709551615, both included, optionally preceded by a + sign and leading zeros.

### Logical constructs

oneOf <constraint script>
  This text constraint matches if one of the text constraints defined in the argument constraint script matches the text. It stops after the first matches and probes the text constraints in the order of definition.
allOf <constraint script>
  This text constraint matches if all of the text constraints defined in the argument constraint script matches the text. It stops after the first match failure and probes the text constraints in the order of definition. Since the schema definition command text also expects all text constraints to match the text constraint, allOf is useful mostly in connection with the oneOf text constraint command.
not <constraint script>
  This text constraint matches if none of the text constraints defined in the argument constraint script matches the text. It stops after the first matching constraint in the constraint script and reports validation error. The text constraints in the constraint script are probed in the order of definition.

### Constraints on processed text value

whitespace (preserve|replace|collapse) <constraint script>
  This text constraint command does white-space (#x20 (space, ' '), #x9 (tab, \t), #xA (linefeed, \n), and #xD (carriage return, \r) normalization to the text value and checks the resulting text with the text constraints of the constraint script argument. The normalization method preserve keeps everything as it is; this is another way to say allOf. The replace normalization method replaces any single white-space character (as above) to a space. The collapse normalization method removes all leading and trailing white-space, and all the other sequences of contiguous white-space are replaced by a single space.
split ?type ?args??<constraint script>

  This text constraint command splits the text to test into a list of values and tests all elements of that list for the text constraints in the evaluated constraint script>.

  The available types are:

  whitespace
    The text to split is stripped of all white space at start and end and splitted into a list at any successive white space.
  tcl tclcmd ?arg ...?
    The text to split is handed to the tclcmd, which is evaluated on global level, appended with every given arg and the text to split as last argument. This call must return a valid Tcl list whose elements are tested.

  The default in case no split type argument is given is whitespace.

strip <constraint script>
  This text constraint command tests all text constraints in the evaluated constraint script> with the text to test stripped of all white space at start and end.

### Various other string properties

fixed value
  The text constraint only matches if the text value is string equal to the given value.
enumeration list
  This text constraint matches if the text value is equal to one element (respecting case and any white-space) of the argument list, which has to be a valid Tcl list.
match ?-nocase? glob_style_match_pattern>
  This text constraint matches if the text value matches the glob style pattern given as argument. It follows the rules of the Tcl [string match] command, see https://www.tcl.tk/man/tcl8.6/TclCmd/string.htm#M35.
regexp expression
  This text constraint matches if the text value matches the regular expression given as argument. https://www.tcl.tk/man/tcl8.6/TclCmd/re_syntax.htm describes the regular expression syntax
length length
  This text constraint matches if the length of the text value (in characters, not bytes) is length. The length argument must be a positive integer or zero.
maxLength length
  This text constraint matches if the length of the text value (in characters, not bytes) is at most length. The length argument must be an integer greater zero.
minLength length
  This text constraint matches if the length of the text value (in characters, not bytes) is at least length. The length argument must be an integer greater zero.
id ?keySpace?
  This text constraint command marks the text as a document wide ID (to be referenced by an idref). Every ID value within a document must be unique. It isn't an error if the ID isn't actually referenced within the document. The optional argument keySpace does all this for a named key space. The key space "" (the empty sting) is another key space then the id command without keySpace argument.
idref ?keySpace?
  This text constraint command expects the text to be a reference to an ID within the document. The referenced ID may appear later in the document, that the reference. Several references within the document to one ID are possible.

## Local key constraints

Document wide uniqueness and foreign key constraints are available with the text constraint commands id and idref. Keyspaces allow for sub-tree local uniqueness and foreign key constraints.

keyspace <names list> <constraint script>
  Any number of keyspaces are possible. A keyspace is either active or not. An inside a constraint script called keyspace with the same name does nothing.

This text constraint commands work with keyspaces:

key <name>
  If the keyspace with the name <name> is not active the constraint always matches. If the keyspace is active, reports error if there is already a key with the value. Otherwise it stores the value as key in this keyspace and matches.
keyref <name>
  If the keyspace with the name <name> is not active always matches. If the keyspace is active then reports error if there is still no key as the value at the end of the keyspace <name>. Otherwise, it matches.

## Recovering

By default the validation engine stops at the first detected validation violation and reports that finding. It does so by return false (and sets, if given, the result variable with an error message) in case the schema command itself is used to validate input. If the schema command is used by a SAX parser or the DOM parser, it does so by throwing error.

If a reportcmd is set this command is called on global level appended with the schema command and an error type as arguments in case a validation violation is detected. Then the validation recovers from the error and continues. For some validation errors the recover strategy can be determined with the script result of the reportcmd.

With a reportcmd (which does not throw error if called) the validation engine will never report validation failure to its caller. The validation engine recovers, continues, and reports the next error (if occuring) and so on until the end of the input. The schema command will return true and the SAX parser and DOM builder will process normally until the end of the input, as if there had not been a validation error.

Please note that this happens only for validation errors. It is not possible to recover from well-formedness errors. If the input is not well-formed, the schema command returns false and sets (if given) the result variable with an error message about the well-formedness error.

If the reportcmd throws error while called by the validation engine then validation stops and the schema command throws error with the error message of the script.

While validating basically three events can happen: an element start tag has to match, a piece of text has to match or an element end tag has to match. The method info vaction called in the recovering script or any script code called from there returns, which event has triggered the error report (MATCH_ELEMENT_START, MATCH_TEXT, MATCH_ELEMENT_END, respectively). While the command walks throu the schema looking whether the event matches other, data driven events (as, for example checking, if any keyref within a keyspace exists) may happen.

Several of the validation error codes, appended as second argument to the reportcmd calls, may happen at more than one kind of validation event. The info vaction method and its subcommands provide information about the current validation event, if called from the report command.

If a structural validation error happens, the default recovering strategy is to ignore any following (or missing) content within the current subtree and to continue with the element end event of the subtree.

Returning "ignore" from the recovering script in case of error type MISSING_ELEMENT recovers by ignoring the failed contraint and continues to match the event further against the schema.

Returning "vanish" from the recover script in case of the error types MISSING_ELEMENT and UNEXPECTED_ELEMENT recovers by ignoring the event.

## Examples

The XML Schema Part 0: Primer Second Edition (https://www.w3.org/TR/xmlschema-0/) starts with this example schema:

    <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

      <xsd:annotation>
        <xsd:documentation xml:lang="en">
         Purchase order schema for Example.com.
         Copyright 2000 Example.com. All rights reserved.
        </xsd:documentation>
      </xsd:annotation>

      <xsd:element name="purchaseOrder" type="PurchaseOrderType"/>

      <xsd:element name="comment" type="xsd:string"/>

      <xsd:complexType name="PurchaseOrderType">
        <xsd:sequence>
          <xsd:element name="shipTo" type="USAddress"/>
          <xsd:element name="billTo" type="USAddress"/>
          <xsd:element ref="comment" minOccurs="0"/>
          <xsd:element name="items"  type="Items"/>
        </xsd:sequence>
        <xsd:attribute name="orderDate" type="xsd:date"/>
      </xsd:complexType>

      <xsd:complexType name="USAddress">
        <xsd:sequence>
          <xsd:element name="name"   type="xsd:string"/>
          <xsd:element name="street" type="xsd:string"/>
          <xsd:element name="city"   type="xsd:string"/>
          <xsd:element name="state"  type="xsd:string"/>
          <xsd:element name="zip"    type="xsd:decimal"/>
        </xsd:sequence>
        <xsd:attribute name="country" type="xsd:NMTOKEN"
                       fixed="US"/>
      </xsd:complexType>

      <xsd:complexType name="Items">
        <xsd:sequence>
          <xsd:element name="item" minOccurs="0" maxOccurs="unbounded">
            <xsd:complexType>
              <xsd:sequence>
                <xsd:element name="productName" type="xsd:string"/>
                <xsd:element name="quantity">
                  <xsd:simpleType>
                    <xsd:restriction base="xsd:positiveInteger">
                      <xsd:maxExclusive value="100"/>
                    </xsd:restriction>
                  </xsd:simpleType>
                </xsd:element>
                <xsd:element name="USPrice"  type="xsd:decimal"/>
                <xsd:element ref="comment"   minOccurs="0"/>
                <xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
              </xsd:sequence>
              <xsd:attribute name="partNum" type="SKU" use="required"/>
            </xsd:complexType>
          </xsd:element>
        </xsd:sequence>
      </xsd:complexType>

      <!-- Stock Keeping Unit, a code for identifying products -->
      <xsd:simpleType name="SKU">
        <xsd:restriction base="xsd:string">
          <xsd:pattern value="\d{3}-[A-Z]{2}"/>
        </xsd:restriction>
      </xsd:simpleType>

    </xsd:schema>

A likely one-to-one translation of that into a tDOM schema definition script would be:

    tdom::schema schema
    schema define {

        # Purchase order schema for Example.com.
        # Copyright 2000 Example.com. All rights reserved.

        defelement purchaseOrder {ref PurchaseOrderType}

        foreach elm {comment name street city state product} {
            defelement $elm text
        }

        defpattern PurchaseOrderType {
            element shipTo ! {ref USAddress}
            element billTo ! {ref USAddress}
            element comment ?
            element items
            attribute orderDate date
        }

        defpattern USAddress {
            element name
            element street
            element city
            element state
            element zip ! {text number}
            attribute country ! {fixed "US"}
        }

        defelement items {
            element item * {
                element product
                element quantity ! {text integer}
                element USPrice ! {text number}
                element comment
                element shipDate ? {text date}
                attribute partNum ! {pattern "^\d{3}-[A-Z]{2}$"}
            }
        }
    }

The RELAX NG Tutorial (http://relaxng.org/tutorial-20011203.html) starts with this example:

    Consider a simple XML representation of an email address book:

    <addressBook>
      <card>
        <name>John Smith</name>
        <email>js@example.com</email>
      </card>
      <card>
        <name>Fred Bloggs</name>
        <email>fb@example.net</email>
      </card>
    </addressBook>

    The DTD would be as follows:

    <!DOCTYPE addressBook [
    <!ELEMENT addressBook (card*)>
    <!ELEMENT card (name, email)>
    <!ELEMENT name (#PCDATA)>
    <!ELEMENT email (#PCDATA)>
    ]>

    A RELAX NG pattern for this could be written as follows:

    <element name="addressBook" xmlns="http://relaxng.org/ns/structure/1.0">
      <zeroOrMore>
        <element name="card">
          <element name="name">
            <text/>
          </element>
          <element name="email">
            <text/>
          </element>
        </element>
      </zeroOrMore>
    </element>

This schema definition script will do the same:

    tdom::schema schema
    schema define {
        defelement addressBook {
            element card *
        }
        defelement card {
            element name
            element email
        }
        foreach e {name email} {
            defelement $e text
        }
    }

## KEYWORDS

Validation, Postvalidation, DOM, SAX