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TGrep-lite search

The TGrep language by Richard Pito formulates queries as patterns that consist of expressions to match tree nodes and relationships defining links or negated links to other tree nodes. Nodes of searched trees are matched either with simple character strings or regular expressions (see sections 1). A complex expression consists of a node expression followed by relationships, as presented in section 2. Possible relationships are illustrated in section 3.

     When you compose a search expression in the box provided, if the search expression uses TGrep-lite syntax, the interface will recognize that and interpret the expression accordingly. If the expression is well-formed but there are no matches in the corpus, the screen shows no change after the “Submit” button is pressed. If the expression is not well-formed, the warning “Not a valid TGrep-lite expression.” appears. In the result page, if you check the “reveal” box and resubmit, a translation of the TGrep-lite expression into XPath syntax is displayed. This allows you to check whether the expression you have composed reflects the search that is intended.

     The TGrep functionality available here is referred to as “TGrep-lite”. This is less expressive than the full TGrep language implemented in the original TGrep program. In particular, the expression of relationships between nodes is limited to the relations detailed in section 3. TGrep-lite is especially weak when compared to enhanced TGrep languages available with TGrep2 and Tregex implementations, notably, missing the ability to express disjunctions of relations. TGrep-lite will also exhibit behaviour distinct from what is expected from TGrep with regards to how nodes are specified (described in section 1). Despite these mentioned limitations, TGrep-lite is the easiest and most accessible way to search the corpus using this on-line interface, and it is a powerful search language.

1   Three kinds of node

TGrep-lite works by rewriting expressions of a modified TGrep language into XPath queries over a database of XML encoded trees. The formatting of the XML requires that the rewrite to XPath distinguishes three different “node” kinds expressed with TGrep-lite node patterns:

• word nodes,
• pre-terminal nodes (that provide information about the word, e.g., the word is a zero element), and
• part-of-speech/phrase-level nodes.

The wild card (“__”) is exceptional in not needing to distinguish its node kind, since it will match all nodes. A simple constant string, such as “abc”, etc., will match word nodes that are the unique string abc. The expression of all other node patterns occurs as the statement of a regular expression with deliminators to determine the kind of the node searched. Specifically:

• a regular expression indicated by surrounding slashes (“/”), such as “/ab/”, will search for word nodes,
• a regular expression indicated by surrounding curly brackets (“{”, “}”), such as “{AB}”, will search for pre-terminal nodes, and
• a regular expression indicated by surrounding square brackets (“[”, “]”), such as “[AB]”, will search for part-of-speech/phrase-level nodes.

     If a simple constant string or deliminated regular expression begins with “!”, the matching process will be complemented. That is, matches will turn into non-matches, and vice-versa. For example, “!abc” will match all words that are not abc, and “![^NP] will match any part-of-speech or phrase-level node that does not start with NP.

     Specified as a string, a regular expression matches a node if there is a part of the node that is matched. For example, “[IP]” matches IP-MAT, IP-ADV, etc. The caret (“^”) anchors the regular expression to the beginning of a matched node, while a dollar sign (“$”) as the last character will anchor the regular expression to the end of a matched node. Use of both the caret and dollar-sign in “[^NP$]” constrains the match to only NP. A word boundary can be stated with “\b”. Thus, while “[^NP]” will match both NP-SBJ and NPR, “[^NP\b]” will match only NP-SBJ. Disjunction can be expressed with the pipe (“|”), and regular expression elements can be grouped with round brackets, such that “[^NP-(SBJ|OB1)]” will find nodes that start with either NP-SBJ or NP-OB1.

     Note that the on-line interface is case insensitive when the node is identified as being either a pre-terminal or part-of-speech/phrase-level node, while being case sensitive for word (terminal) nodes.

2   Constructing expressions

TGrep-lite expressions are composed of a node pattern followed by the relationships the node pattern participates in. Because word information serves as content of the same node under the XML encoding as pre-terminal node information, it becomes necessary if you wish to match the combination of a particular word with a particular pre-terminal node that the “==” (equals) relation serves to connect this information about the same underlying node. For example, the following will find instances of words that contain “tuti” with the “PHON” pre-terminal tag.

/tuti/ == {PHON}

The following example,

([IP] < [PP]) << [IP]

will match an IP node which immediately dominates a PP node and which dominates an IP node. Note the parenthesis to ensure that the second relationship “<< [IP]” refers to the first IP and not to the PP. As another example,

[IP] < ([PP] << [IP])

will match an IP which immediately dominates a PP which in turn dominates some IP.

     The first node in a pattern or the first node following a left parenthesis is a “master” node which is related to the relationships to its right. Thus, a TGrep-lite pattern consists of a master node for the entire query followed by a series of relationships to other nodes that can themselves with parenthesis form master nodes with relationships to yet other nodes. In the first example above only the first [IP] is a master node, while in the second example both the first [IP] and the [PP] are master nodes.

3   Relationships between nodes

Relationships define connections between the master node (being defined) and other nodes. There is a complete pairing of forward and backward links, allowing for flexibility in choosing what is the master node. Notable relationships are:

  A << B     A dominates (is an ancestor of) B
  A >> B     A is dominated by (is a descendant of) B
  A < B      A immediately dominates (is the parent of) B
  A > B      A is immediately dominated by (is the child of) B
  A .. B     A precedes B
  A ,, B     A follows B
  A . B      A immediately precedes B
  A , B      A immediately follows B
  A $ B      A is a sister of and not equal to B
  A $.. B    A is a sister of and precedes B
  A $,, B    A is a sister of and follows B
  A $. B     A is a sister of and immediately precedes B
  A $, B     A is a sister of and immediately follows B
  A $, B     A is a sister of and immediately follows B
  A == B     A and B are the same node
  A <<, B    B is a leftmost descendant of A
  A <<- B    B is a rightmost descendant of A
  A >>, B    A is a leftmost descendant of B
  A >>- B    A is a rightmost descendant of B
  A <1 B     B is the 1st child of A
  A >1 B     A is the 1st child of B
  A <-1 B    B is the last child of A
  A >-1 B    A is the last child of B
  A <, B     B is the first child of A (synonymous with A <1 B)
  A >, B     A is the first child of B (synonymous with A >1 B)
  A <- B     B is the last child of A (also synonymous with A <-1 B)
  A >- B     A is the last child of B (also synonymous with A >-1 B)
  A <: B     B is the only child of A
  A >: B     A is the only child of B
  A <<: B    A dominates B via an unbroken chain (length > 0) of unary branches
  A >>: B    A is dominated by B via an unbroken chain (length > 0) of unary branches

     The following presents pictures grouping some of the above relationships as forward and backward links:

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C << __ (dominates, is an ancestor of)

__ >> C (is dominated by, is a descendant of)

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E >> __ (is dominated by, is a descendant of)

__ << E (dominates, is an ancestor of)

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C > __ (immediately dominates, is the parent of)

__ < C (is immediately dominated by, is the child of)

---

I .. __ (precedes)

__ ,, I (follows)

---

I ,, __ (follows)

__ .. I (precedes)

---

I . __ (immediately precedes)

__ , I (immediately follows)

---

I , __ (immediately follows)

__ . I (immediately precedes)

---

F $ __ (sister)

__ $ F (sister)

---

E $.. __ (sister and precedes)

__ $,, E (sister and follows)

---

E $. __ (sister and immediately precedes)

__ $, E (sister and immediately follows)

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Negation

An exclamation mark (!) can be placed immediately before any relationship to negate it. Thus, A !.. B means that A is not followed by B.

Special note on what is matched

TGrep-lite returns the match for the left-most element in the search pattern. The following pattern matches PPs that are immediately dominated by an IP that dominates an IP:

[PP] > ([IP] < [IP])

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Search results are listed in groups of up to twenty five entries, each with highlighted portions corresponding to the focus of the query. Immediately following each entry is a link to the tree for that entry in the form of the ID number of that entry. Following the link opens a tree view for the result, with highlighted nodes corresponding to the focus of the search. When appropriate, there is a down arrow to click for moving to the next twenty five results, and an up arrow for moving back.

In addition, there is an open text area that contains the pattern for the search. This gives the opportunity to see and also edit the search query. Clicking the “Submit” button re-submits the possibly edited search.

At the page end, there is the option to download results for searches with results of 2000 items or less. There are three possible forms in which search results can be downloaded: basic text format, bracket format, and Alpino XML format. All formats include the text and ID number of each entry. Bracket format and Alpino XML format include all the syntactic information encoded for each entry. Each line of text with the “basic text format” is a tab separated numbered entry, and the number of the last entry is equal to the number of results for the search.

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