Aiming for a balance of linguistic precision and broad coverage, the English Resource Grammar (ERG) includes detailed analyses of punctuation and a wide variety of 'text-level' phenomena (e.g. various formats for temporal and numeric expressions). The grammar makes specific assumptions about tokenization, and for the successful application of the grammar it is important to understand and respect these assumptions. In early 2009, the ERG approach to tokenization has undergone a major revision, and this page aims to spell out some of the basic assumptions, specific decisions made, and technology used in preparing input text for parsing with the ERG. When using the ERG to parse ‘raw’ string inputs, for satisfactory results on non-vanilla inputs, it is necessary to apply the ERG tokenization rules, e.g. turn on support for REPP in engines like PET or ACE.
This page was predominantly authored by StephanOepen, who jointly with DanFlickinger developed the current ERG approach to tokenization. As of early 2009, Stephan is the maintainer of the ERG tokenizer and token mapping rules. Please do not make substantial changes to this page unless you (a) are reasonably sure of the technical correctness of your revisions and (b) believe strongly that your changes are compatible with the general design and recommended use patterns for the ERG, and of course with the goals of this page.
Pre-Processing and Initial Tokenization
This section documents tokenization and a handful of other surface-level decisions. Technically speaking, when parsing with the ERG and PET (which is the reference setup for production use), the parser takes as its input a lattice of tokens, each a structured object (aka typed feature structure). Please see the PetInput page for additional background. In this view, string-level pre-processing and initial tokenization is the process of mapping a 'flat' string into a token lattice. For further technical background and an example demonstrating a broad range of tokenization aspects, please see the page ErgTokenization/ComplexExample.
In the standard setup for the ERG, this task is solved by means of so-called REPP (Regular Expression Pre-Processor) modules, which are included with the ERG sources (in the rpp/ subdirectory); for general background on the technology, please see the ReppTop page. The REPP modules provided by the ERG can be configured in various ways, to accommodate different input conventions, i.e. variation in punctuation and markup conventions used in texts from various sources. As of mid-2010, these REPP modules have stabilized to a certain degree but remain to be documented (beyond the generous use of comments in the REPP source files). In the following, we document the normalized result of string-level pre-processing, i.e. the expected input to the ERG (and result of the application of a set of REPP modules).
For compatibility with existing tools, specifically taggers trained on the Penn Treebank (PTB), we assume a PTB-like tokenization in pre-processing. The ERG internally (still) analyzes most punctuation marks as pseudo-affixes (rather than as separate tokens, as in the PTB). To accomodate any discrepancies, the grammar includes token mapping rules to adjust (i.e. correct) externally supplied tokenization (see the ChartMapping page for general background); specifically, punctuation marks will be re-combined with preceding or following tokens, reflecting standard orthographic convention.
The REPP pre-processing modules included with the ERG are inspired by the PTB tokenizer.sed script and by and large yield quite similar results (with a number of extensions going beyond 7-bit ASCII strings, as discussed below). To actually tokenize (following PTB principles), we need to do more than just break at whitespace. Some punctuation marks give rise to token boundaries, but not all. Also, inputs (in the 21st century) may contain some amount of mark-up, where XML character references for example have become relatively common. Full UniCode support in the toolchain now makes it possible to represent a much larger range of characters, e.g. various types of quotes and dashes. In general, we aim to map mark-up to corresponding UniCode characters, where appropriate, and typically analyze those in parsing.
However, the original tokenizer.sed actually does not always yield the exact tokenization found in the PTB. For example, the script unconditionally separates a set of punctuation or other non-alphanumeric characters (e.g. & and ! that may be part of a single token (say in acronyms like AT&T or URLs). We aim to do better than the original script, here, conditioning token boundaries on (transitively) adjacent whitespace. See the examples discussed below for details.
A Running Example
To exemplify the above basic principles, consider the following sample input:
The shipment, 'chairs', arrived.
This will be tokenized into a total of nine tokens, i.e. each of the punctuation marks will form a token in its own right. In REPP, each token will be annotated with so-called 'characterization', i.e. a range of character indices into the original string (allowing one to recover the distinction between immediate adjacency between two consecutive tokens vs. intervening whitespace). Thus, in the so-called YY input format to PET (see the PetInput page for background), our example would be represented as follows:
(42, 0, 1, <0:3>, 1, "The", 0, "null") (43, 1, 2, <4:12>, 1, "shipment", 0, "null") (44, 2, 3, <12:13>, 1, ",", 0, "null") (45, 3, 4, <14:15>, 1, "‘", 0, "null") (46, 4, 5, <15:21>, 1, "chairs", 0, "null") (47, 5, 6, <21:22>, 1, "’", 0, "null") (48, 6, 7, <22:23>, 1, ",", 0, "null") (49, 7, 8, <24:31>, 1, "arrived", 0, "null") (50, 8, 9, <31:32>, 1, ".", 0, "null")
Purely for PTB compatibility, albeit at the expense of linguistic adequacy, contracted negations are also separated into multiple tokens, e.g. ca n’t, do n’t, etc.
In naturally occuring texts, there is a wide variety of conventions for representing quotation marks. Much like in the PTB, the ERG expects its inputs (after pre-processing) to distinguish between opening (aka left) and closing (aka right) quotes. Note how REPP in the above example turns the straight single quotes (so-called 'typewriter quotes') into directional UniCode characters, i.e. ‘ (U+2018) and ’ (U+2019). This is a prerequisite to parsing success for this example with the ERG, i.e. in a setup by-passing the REPP layer (and allowing straight quotes in the raw inputs), quote disambiguation (based on transitive adjacency to whitespace) needs to be supplied by the tokenizer.
Note that, unlike the PTB, the ERG uses directional UniCode quotes instead of the ancient LaTeX-like ASCII convention adapted for the original PTB, i.e. interpreting a backquote (aka grave accent) as a left quotation mark, and then reserving the straight quote for right quotation marks. The same would be true for double quotes, i.e. instead of the double-character representations used in the PTB (`` and ''), the ERG expects genuine opening and closing quotation marks, viz. “ (U+201c) and ” (U+201d); see Wikipedia for a more general discussion of quotation marks in English.
In practice, today we observe three mutually incompatible conventions in electronic texts: (a) the PTB- or LaTeX-like abuse of grave accents (e.g. `quote'); (b) use of plain ASCII, not attempting to differentiate left and right quotes (e.g. 'quote'); and (c) modern, high-quality typesetting, using genuine directional quotation marks (e.g. ‘quote’. Variants (a) and (b) are both frequently found in electronic texts that were 'typed' rather than 'typeset', i.e. originally written without the goal of professional typography. Variant (c), on the other hand, is more common in texts derived from carefully typeset sources, for example scholarly manuscripts (for example when extracting a text stream from an XML or PDF file). While (a) and (c) distinguish opening from closing quotes, (b) does not; yet, the distinction is reliably recoverable at the string level, i.e. in pre-processing (and not necessarily later on).
These are some of the reasons for the ERG to assume 'normalized' inputs, where different configurations of REPP modules are provided to properly handle the above conventions. Furthermore, the PTB decision to use a double-character sequence to represent a single glyph (viz. opening and closing quotes) is problematic in several ways: it complicates regular expression writing; it potentially skews character offset computations; and most importantly it makes inaccessible a legitimate character sequence, viz. two consecutive single quotes (or apostrophes); in some bio-medical journals, for example, it is quite common to distinguish identifiers A', A'', A''', and so on. For further subtle points of quotation marks, please see the discussion by Markus Kuhn, one of the UniCode pioneers.
The apostrophe, as used for contractions and possessives in English, is typographically (almost) never distinguished from a single closing quote. Because the possessive will be spelled as a sole apostrophe when following a word ending in -s (as for example in Abrams’), and also for reasons of syntax (the possessive attaches to complete NPs; its homograph, the contracted present-tense, third-person, singular form of the auxiliary, acts as the head of VP), possessives always need to be tokens of their own in pre-processed inputs to the ERG, e.g. Browne ’s chair, Abrams ’ chair, or Browne ’s efficient.
In practice, both forms of the single quote discussed above are found as apostrophes in electronic texts, i.e. the 'typewriter' apostrophe (typically in variants (a) and (b) above), as well as the UniCode apostrophe (U+2019), mostly in variant (c). Seeing what we argued so far, the grammar should probably only support the normalized UniCode apostrophe, e.g. do n’t, o’clock, or Browne ’s, rather than do n't, o'clock, and Browne 's. As of July 2010, this is unfortunately not yet the case. For common contractions and the possessives, both variants are currently supported; however, for rare words containing apostrophes one or the other variant may be missing from the lexicon, e.g. a UniCode version of the multi-word name Ayer 's Rock (note the tokenization into three units, which follows from the seemingly possessive-like structure).
Other Diversions from the PTB
For reasons similar to those given in the case of double quotation marks above, REPP also normalizes a few more multi-character ASCII representations of UniCode glyphs, viz. em dashes (---; U+2014), en dashes (--; U+2013), and ellipsis (...; U+2026).
Light-Weight Named Entitities