Grammar Specs

UNL grammars are sets of rules for translating UNL expressions into natural language (NL) sentences and NL sentences into UNL expressions. They are normally unidirectional, i.e., the UNL-ization grammar (NL-to-UNL) is different from the NL-ization grammar (UNL-to-NL), even though they share the same basic syntax.

Basic symbols

Basic symbols used in UNL grammar rules

Symbol	Definition	Example
^	not	^a = not a
{ | }	or	{a|b} = a or b
%	index for nodes, attributes and values	%x (see below)
#	index for sub-NLWs	#01 (see below)
=	attribute-value assignment	POS=NOU
!	rule trigger	!PLR
&	merge operator	%x&%y
?	dictionary lookup operator	?[a]
“ “	string	"went"
[ ]	natural language entry (headword)	[go]
[[ ]]	UW	[[to go(icl>to move)]]
( )	node	(a)
//	regular expression	/a{2,3}/ = aa,aaa

The differences between "", [] and [[]]

Double quotes are always used to represent strings: "a" will match only the string "a"

Simple square brackets are always used to represent natural language entries (headwords) in the dictionary: [a] will match the node associated to the entry [a] retrieved from the dictionary, no matter its current realization, which may be affected by other rules (the original [a] may have been replaced, for instance, by "b", but will still be indexed to the entry [a])

Double square brackets are always used to represent UWs: [[a]] will match the node associated to the UW [[a]]

Predefined values (assigned by default)

SCOPE - Scope

SHEAD - Sentence head (the beginning of a sentence)

STAIL - Sentence tail (the end of a sentence)

CHEAD - Scope head (the beginning of a scope)

CTAIL - Scope tail (the end of a scope)

TEMP - Temporary entry (entry not found in the dictionary)

DIGIT - Any sequence of digits (i.e.: 0,1,2,3,4,5,6,7,8,9)

Basic concepts

Nodes

A node is the most elementary unit in the grammar. It is the result of the tokenization process, and corresponds to the notion of "lexical item", to be represented by dictionary entries. At the surface level, a natural language sentence is considered a list of nodes, and a UNL graph a set of relations between nodes. Any node is a vector (one-dimensional array) containing the following necessary elements:

• a string, to be represented between "quotes", which expresses the actual state of the node;
• a headword, to be represented between [square brackets], which expresses the original value of the node in the dictionary;
• a UW, to be represented between [[double square brackets]], which expresses the UW value of the node;
• a feature or set of features, which express the features of the node;
• an index, preceded by the symbol %, which is used to reference the node;

Examples of nodes are

• ("ing") (a node making reference only to its actual string value)
• ([book]) (a node making reference only to its headword,i.e., its original state in the dictionary)
• ([[book(icl>document)]]) (a node making reference only to its UW value)
• (NUM) (a node making reference only to one of its features)
• (POS=NOU) (a node making reference only to one of its features in the attribute-value pair format)
• (%x) (a node making reference only to its unique index)
• ("string",[headword],[[UW]],feature1,feature2,...,attribute1=value1,attribute2=value2,...,%x) (complete node)

Properties of nodes

Nodes are enclosed between (parentheses)

("a") is a node

"a" is not a note

The elements of a node are separated by comma

("a",[a],[[a]],A,B,A=C,%a)

The order of elements inside a node is not relevant.

("a",[a],[[a]],A,B,A=C,%a) is the same as ([[a]],B,A,"a",[a],A=C,%a)

Nodes may have one single string, headword, UW and index, but may have as many features as necessary

("a","b") (a node may not contain more than one string)

([a],[b]) (a node may not contain more than one headword)

([[a]],[[b]]) (a node may not contain more than one UW)

(%a,%b) (a node may not contain more than one index)

(A,B,C,D,...,Z) (a node may contain as many features as necessary)

A node may be referred by any of its elements

("a") refers to all nodes where actual string = "a"

([a]) refers to all nodes where headword = [a]

([[a]]) refers to all nodes where UW = [[a]]

(A) refers to all nodes having the feature A

("a",[a],[[a]],A) refers to all nodes having the feature A where string = "a" and headword = [a] and UW = [[a]]

Nodes are automatically indexed according to a position-based system if no explicit index is provided (see Index)

("a")("b") is actually ("a",%01)("b",%02)

Regular expressions may be used to make reference to any element of the node, except the index

("/a{2,3}/") refers to all nodes where string is a sequence of 2 to 3 characters "a"

([/a{2,3}/]) refers to all nodes where headword is a sequence of 2 to 3 characters "a"

([[/a{2,3}/]]) refers to all nodes where UW is a sequence of 2 to 3 characters "a"

(/a{2,3}/) refers to all nodes having a feature that is a sequence of 2 to 3 characters "a"

Nodes may contain disjoint features enclosed between {braces} and separated by comma

({A|B}) refers to all nodes having the feature A OR B

Node features may be expressed as simple attributes, or attribute-value pairs

(MCL) - feature as an attribute: refers to all nodes having the feature MCL

(GEN=MCL) - feature as an attribute-value pair, which is the same as (GEN,MCL): refers to all nodes having the features GEN and MCL.

Attribute-value pairs may be used to create co-reference between different nodes (as in agreement):

(%x,GEN)(%y,GEN=%x) - the value of the attribute GEN of the node %x is the same of the attribute GEN of the node %y (see Index)

Relations

In order to form a natural language sentence or a UNL graph, nodes are inter-related by relations. In the UNL framework, there can be three different types of relations:

• the linear relation L expresses the surface structure of natural language sentences
• syntactic relations express the deep (tree) structure of natural language sentences
• semantic relations express the structure of UNL graphs

Properties of relations

The linear relation is always binary and is represented in two possible formats

• L(%x;%y), where L is the invariant name of the linear relation, and %x and %y are nodes; or
• (%x)(%y)

Syntactic relations are not predefined, although we have been using a set of binary relations based on the X-bar theory.

Semantic relations constitute a predefined and closed set that can be found here.

Syntactic and semantic relations are represented in the same way

• rel(%x;%y), where "rel" is the name of the relation, %x is the source node, and %y is the target node

Arguments of linear, syntactic and semantic relations are not commutative.

The order of the elements in a relation affects the result:

(%x)(%y) is different from (%y)(%x)

relation(%x;%y) is different from relation(%y;%x)

Linear and semantic relations are always binary; syntactic relations may be n-ary

L(%x;%y) - linear relation

agt(%x;%y) - semantic relation

VH(%x) - unary syntactic relation

VC(%x;%y) - binary syntactic relation

XX(%x;%y;%z) - possible ternary syntactic relation

Inside each relation, nodes are isolated by semicolon (;).

VC(%x;%y)

VC(%x,%y)

Inside each relation, nodes may be referenced by any of its elements, isolated by comma (,)

("a")([b]) - linear relation between a node where string = "a" and another node where headword = [b]

L([[c]];D) - linear relation between a node where UW = [[c]] and another node having the feature D

VC(%a;%b) - syntactic relation between a node where index = %a and another node where index = %b

agt("a",[a],[[a]],A;"b",[b],[[b]],B) - semantic relation between a node having the feature A where string = "a" AND headword "a" AND UW = [[a]] AND another node having the feature B where string = "b" AND headword = [b] AND UW = [[b]]

Relations may be conjoined through juxtaposition

("a")("b")("c") - two linear relations: one between ("a") and ("b") AND other between ("b") and ("c")

agt(%x;%y)obj(%x;%z) - two semantic relations: one between (%x) and (%y) AND other between (%x) and (%z)

VC([a];[b]),VC([a];[c]) - conjoined relations must not be isolated by comma

Relations may be disjoined through {braces}

{("a")|("b")}("c") - either ("a")("c") or ("b")("c")

{agt(%x;%y)|exp(%x;%y)}obj(%x;%z) - either agt(%x;%y)obj(%x;%z) or exp(%x;%y)obj(%x;%z)

Syntactic and semantic relations may be replaced by regular expressions

/.{2,3}/(%x;%y) - any relation made of two or three characters between %x and %y

Hyper-nodes

Nodes may contain one or more relations. In this case, they are said to be "hyper-nodes", and represent scopes or sub-graphs. As any node, hyper-nodes contain a string, a headword, a UW, an index and features, of which the internal relations are a special type. Examples of hyper-nodes are the following:

• (("a")("b")) - a hyper-node containing a linear relation between the nodes ("a") and ("b")
• (VC(%x;%y)VA(%x;%z)) - a hyper-node containing two syntactic relations: VC(%x;%y)AND VA(%x;%z)
• (agt([a];[b])obj([a];[c])) - a hyper-node containing two semantic relations: agt([a];[b]) AND obj([a];[c])
• (([kick],V)([the],D)([bucket],N),V,NTST) - a hyper-node having the features N and NTST and containing two linear relations: one between the nodes ([kick],V) and ([the],D), and other between ([the],D) and [bucket],N)
• (([kick],V)([the],D)([bucket],N),"kick the bucket",[[die]],V,NTST) - the same as before, except for the fact that the hyper-node has string = "kick the bucket" and UW = [[die]]

Hyper-nodes may also contain internal hyper-nodes:

• ((("a")("b"))("c")) - a hyper-node containing a linear relation between the hyper-node (("a")("b")) and the node ("c")

Properties of hyper-nodes

As any node, hyper-nodes are expressed between (parentheses)

(("a")("b"))

As any node, hyper-nodes may have one single string, one single headword and one single UW, but may have as many features and internal relations as necessary

(([kick],V)([the],D)([bucket],N),"kick the bucket",[kick the bucket],[[die]],V,NTST)

As any node, hyper-nodes may be referenced by any of its elements, including internal relations

(([kick],V)) - refers to any hyper-node containing the node ([kick],V)

(([the],D)([bucket],N)) - refers to any hyper-node containing a linear relation between ([the],D) AND ([bucket],N)

(([kick],D),([bucket],N)) - refers to any hyper-node containing the nodes ([kick],V) AND ([bucket],N)

When a hyper-node is deleted, all its internal relations are deleted as well

(([kick],V)([the],D)([bucket],N)):=; (the hyper-node is deleted, as well as the relations ([kick],V)([the],D) AND ([the],D)([bucket],N))

Hyper-relations

Relations may have relations as arguments. In this case, they are said to be "hyper-relations". Examples of hyper-relations are the following:

• XP(XB(%a;%b);%c) - a syntactic relation XP between the syntactic relation XB(%a;%b) and the node %c
• and(agt([a];[b]);agt([a];[c])) - a semantic relation "and" between the semantic relations agt([a];[b]) AND agt([a];[c])

Properties of hyper-relations

A hyper-relation may have one single relation as each argument

• XP(XB(%a;%b);%c) - the source argument of the hyper-relation XP is a relation
• XP(%a;XB(%b;%c)) - the target argument of the hyper-relation XP is a relation
• XP(VC(%a;%b);VA(%a;%c)) - the source and the target argument of the hyper-relation XP are relations
• XP(VC(%a;%b)VA(%a;%c);VS(%a;%d)) - a hyper-relation may not have more than one relation as one single argument (in this case, the hyper-relation XP contained two relations as the source argument)

Relations do not have strings, UWs, headwords or any features

• XP(XB(%a;%b),"ab",[ab],[[ab]],A,B;%c) (the relation XB(%a;%b) may not have strings, UWs, headwords or any features)

Types of rules

In the UNL Grammar there are three basic types of rules:

Normalization Rules

(main article: N-Rules Used to normalize the natural language input and to segment natural language texts into sentences.

Transformation rules

(main article: T-Rules Used to generate natural language sentences out of UNL graphs and vice-versa.

Disambiguation rules

(main article: D-rules Used to improve the performance of transformation rules by constraining their applicability.

The Segmentation Rules and Transformation Rules follow the very general formalism

α:=β;

where the left side α is a condition statement, and the right side β is an action to be performed over α.

The Disambiguation Rules, which were directly inspired by the UNL Centre's former co-occurrence dictionary and knowledge base, follows a slightly different formalism:

α=P;

where the left side α is a statement and the right side P is an integer from 0 to 255 that indicates the probability of occurrence of α.