PRE-PROCESSING TASKS FOR RULE-BASED
ENGLISH-KOREAN MACHINE TRANSLATION SYSTEM
Sung-Dong Kim
Dept. of Computer Engineering, Hansung University, 389 Samseon-dong, Seongbuk-gu, Seoul, Republic of Korea
Keywords: Rule-based machine translation, Natural language processing, Pre-processing.
Abstract: This paper presents necessary pre-processing tasks for practical English-Korean machine translation. The
pre-processing task consists of a problem that requires pre-processing and a solution for the problem. There
are many differences between English and Korean, so it is difficult to resolve the differences using parsing
and transfer rules. Also, source sentences often include non-word elements, such as parentheses, quotation
marks, and list markers. In order to resolve the differences efficiently and make source sentences
appropriate to translation system by arranging them, we propose pre-processing for source sentences. This
paper studies various pre-processing tasks and classifies into several groups according to the time when the
tasks are performed in English-Korean machine translation system. In experiment, we show the usefulness
of the defined pre-processing tasks for generating better translation results.
1 INTRODUCTION
Recent English-Korean machine translation systems
generate good translation for relatively short
sentences. But there are problems that a practical
English-Korean machine translation system must
solve. It is difficult to translate long sentences and
sentences with special patterns. In rule-based
translation, context-free grammar is generally used
to represent English syntactic structures. The
grammar has limitation to express structures for long
sentences consisting of comma-separated sub-
sentences and for sentences with special patterns.
Especially, the syntactic analysis of sentences with
commas is very difficult. It is difficult to try to cover
those sentences using syntactic rules. Also, there are
many differences between English and Korean, so it
is difficult to resolve the differences using parsing
and transfer rules. An idiom-based translation
approach (Yoon, 1993) is adopted to overcome the
differences, where fixed format idioms and phrasal
idioms are effective in generating readable and
meaningful translation results. Further, they try to
translate sentences with special patterns using
extended idioms (Kim and Kim, 1998). But the
idiom translation approach may cause the side
effects in idiom recognition that interfere parsing
and result in wrong translations. In practical
English-Korean translation, source sentences often
include non-word elements, such as parentheses,
quotation marks, list markers, and etc. These non-
word elements make the syntactic analysis difficult,
so they are processed properly before the normal
translation process.
This paper studies a pre-processing as a method
of solving the above problems in rule-based English-
Korean machine translation. The target of pre-
processing in this paper is an input source sentence
from plain documents, rather than formatted
documents like HTML ones. The system has rules
for lexical analysis, parsing and transfer. It adopts
idiom-translation approach to resolve differences
between two languages and uses partial parsing
method by segmenting source sentences to
efficiently translate long sentences. In this paper, we
search problems that require pre-processing during
the analysis steps: lexical analysis, sentence
segmentation, parsing, and transfer. Also, we present
the solutions for the problems. A pre-processing
task consists of a problem and a solution. We
classify the pre-processing tasks into groups
according to the time when the tasks are performed.
Section 2 briefly surveys other works for pre-
processing in English-Korean machine translation.
Section 3 presents the pre-processing tasks and their
classification. Section 4 shows how many sentences
will be benefited by the defined pre-processing tasks.
Section 5 concludes the paper with further works.
257
Kim S..
PRE-PROCESSING TASKS FOR RULE-BASED ENGLISH-KOREAN MACHINE TRANSLATION SYSTEM .
DOI: 10.5220/0003151702570262
In Proceedings of the 3rd International Conference on Agents and Artificial Intelligence (ICAART-2011), pages 257-262
ISBN: 978-989-8425-40-9
Copyright
c
 2011 SCITEPRESS (Science and Technology Publications, Lda.)

2 RELATED WORKS
English-Korean machine translation (EKMT) treats
two languages that are very different in nature. The
differences must be solved for accurate translation.
In building an EKMT system, pre-processing is
useful in solving the differences and will be applied
in various positions through the translation steps.
English has hyphenated words. (Yuh et al., 1997)
proposed translation method for hyphenated words
which uses morphological analysis and considers
part-of-speech sequence. In (Yuh et al. 1996), they
defined the functions of a pre-processor for EKMT.
They presented sentence splitting in a given
document, words identification (hyphenated words,
pronoun, abbreviations, and special symbols),
normalizing upper/lower case letters and recognition
of composition words (multi-word numeric
expression, geographic names, organization names)
as major functions of the pre-processor. The above
studies were for word-level pre-processing problems
and the pre-processor must be positioned before
normal translation process.
In translation of long English sentences, sentence
segmentation and partial parsing were used (Kim et
al., 2001). Also, (Kim, 2008) presented comma
rewriting for accurate analysis of long sentences
consisting of comma-separated sub-sentences. These
are pre-processing of source sentences for efficient
and accurate translation of long sentences. They
were for phrase/sentence-level problems.
This paper considers above studies, searches
necessary pre-processing problems, and rearranges
them with their solutions. The pre-processing
problems in this paper cover both word-level and
phrase/sentence-level problems.
3 PRE-PROCESSING TASKS
This section briefly explains our own English-
Korean machine translation (EKMT) system,
SmarTran, and presents necessary pre-processing
tasks in the translation process. Some pre-processing
tasks require corresponding post-processing. We
also describe post-processing tasks in this section.
3.1 SmarTran System
Figure 1 shows the logical structure of the SmarTran.
Given a source sentence, lexical analysis is done
using English lexical dictionary and rules. Necessary
lexical information is collected for each word in the
sentence. Using the information, sentence is split
into several segments for efficient parsing. Each
segment is parsed in partial parsing step using idiom
recognition and English syntactic rules. Then a
global sentence structure is built using the partial
parsing results. A transfer is performed on the
resulting structure using English-Korean transfer
dictionary and transfer rules. The transferred
structure is passed to the generation step which
generates corresponding target sentence using
Korean generation dictionary and rules.
Figure 1: Logical structure of SmarTran system.
3.2 Classification of Pre-processing
Tasks
3.2.1 Tasks before Lexical Analysis
Some English sentences include non-word elements
such as parentheses, quotation marks, hyphens, list
markers, semicolons, and colons. These elements
separate a sentence into several translation units.
The translation units must be identified before
translation process. Also, sentences may include
special words such as number (with unit word),
composition words, and meaningless words. Some
special patterns must be identified to solve
differences between source and target languages. We
consider 8 pre-processing problems that can be
Sourcesentence
Lexicalanalysis
Sentencesegmentation
Partialparsing
Globalstructure
Transfer
Generation
Target sentence
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handled without part-of-speech and other lexical
information.
First, a sentence may be split into translation
units by semicolon, colon, and bar. The units must
be translated independently.
Second, parts of a sentence enclosed by a pair of
single or double quotation marks are separate
translation units. Parentheses and angle brackets also
enclose some parts of a sentence that are also
translation units. The enclosed parts must be
separated and be translated independently, but they
are also elements of other translation unit. Figure 2
shows two examples. The enclosed part Q1 must be
translated together with a given sentence as in TU1,
while P1 can be translated independently. In case of
P1, the target word for TU2 must be identified for
post-processing in which the translation result of
TU2 is appended to the translation of the target word.
Figure 2: Examples of sentences with translation unit
enclosed by double quotation marks.
Third, some sentences have head marks leading
list. When the mark is a symbol, it is easily removed.
The mark must be recognized and removed when it
is a digit or alphabetical digit (ex: i, ii, I, II, …).
Fourth, words abbreviated by apostrophe must be
restored for facilitating lexical analysis. For example,
“don’t” must be converted to “do not”. We build
dictionary for such words.
Fifth, words representing numbers must be
analyzed to know whether they are ordinal or
cardinal. Also, we must identify the combination of
number and unit words. In this case, the two words
must be combined. For the purpose, we need
information about unit and number words.
Sixth, we must identify composite words. Two or
more words play a role of a one word noun, verb,
adverb, preposition, or conjunction. Composite
nouns can be translated by idiom translation method,
while composite verbs, adverb, prepositions, and
conjunctions can be collected and combined into one
word. We need the list of composition words with
their translations.
Seventh, some special patterns must be handled.
For example, sentences including [~ so that ~]
pattern can be rewritten into [~, so ~], and the
rewritten sentences are easier to be analyzed. We
collect patterns requiring sentence rewriting, and
build corresponding rewriting patterns.
Eighth, phrases expressing date must be
identified and treated as one word. Also the phrases
are translated in separate post-processing for date
translation. There are several patterns for
representing date. We collect the patterns to be used
in identification and build corresponding translation
patterns for translation.
3.2.2 Tasks after Lexical Analysis
Some pre-processing problems need lexical
information such as part-of-speech, part-of-speech
probability, and etc. We present 5 pre-processing
tasks as followings.
First, sentences may include phrases expressing
human name and his age. The phrases must be
combined and treated as one word during lexical and
syntactic analysis. It needs corresponding post-
processing in which the combined phrases are
translated into Korean.
Second, geographical names consisting of
pronouns and comma must be combined. For
example, in sentence “I lived in Brynmawr, PA.”,
“Brynmawr, PA.” is combined, so the phrase can be
translated as one word. In order to solve above two
problems, we need to know whether a word is
pronoun or not.
Third, some sentences start with adverb or
adverbial phrase which modifies the following
sentence. The modifier can be separated, which can
reduce the parsing complexity.
Fourth, some sentences include patterns for
which the translation is difficult. Such patterns
include [not only ~ but (also) ~], [insist ~ that ~
VERB (base form) ~], [no sooner had ~ than ~], and
so on. We need lexical information to match ‘~’
parts in the patterns. For the patterns, we adopt
rewriting method using rewriting patterns. In this
pre-processing, the sentences matched with the
defined patterns are rewritten as directed by the
corresponding patterns. The corresponding patterns
have compatible meanings and forms that are easier
to be analyzed in the rule-based framework. For
example, [no sooner had ~ than ~] pattern has [as
soon as ~, ~] as its corresponding pattern.
Fifth, we consider comma rewriting for
preventing non-constituent segments from occurring
by segmentation. For example, in “I need small, fast
"Next year, I may evaluate it a little closer," said
Stan Guest, an uninsured farmer.
TU1: Q1, said Stan Guest, an uninsured farmer
TU2 = Q1: Next year, I may evaluate it a little closer
During the eighth five-year plan period (from 1991 to
1995), the reform successfully completed.
TU1: During the eighth five-year plan period, the
reform successfully completed
TU2=P1: from 1991 to 1995
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computer,” the comma can be rewritten into “and,”
resulting in “I need small and fast computer.” This
comma rewriting requires information about comma
usage (Kim and Park, 2006).
3.2.3 Tasks after Sentence Segmentation
Source sentences with commas can be split by
commas resulting in several segments (Kim et al.,
2001). There are pre-processing tasks that can be
done after sentence segmentation.
First, we search special patterns within each
segment. It is difficult to get accurate translation for
sentences with such patterns as [so ~ that ~], [it BE-
verb ~ ADJ that ~] and [it BE-verb ~ that ~]. The
patterns have information about split position and
how to combine the parsing results of split sub-
segments. We split a segment including such
patterns into two sub-segments. Each sub-segment is
parsed independently and the parsing results are
combined based on the combination rules for the
patterns.
Second, there are several patterns for verb “say.”
When “say” verb has object element enclosed by a
pair of double quotation marks, the order of words in
the sentence may be different from normal sentences.
It is difficult to parse such sentences in rule-based
framework, so sentence elements repositioning is
required. Figure 3 shows two examples. The first
example is from figure 2. After translation units are
identified, the units are rearranged. Actually,
sentence elements repositioning is a rearrangement
of translation units. This repositioning results in a
rearranged sentence in which order of words was
described by the existing syntactic rules. Through
the repositioning, we identify the role of the
segments and this information is used to generate
global sentence structure from partial parsing results
of each segment.
Figure 3: Examples of sentences elements repositioning.
Third, some phrases or clauses can be inserted
which play roles of adverb, modifier, and etc. The
insertion is generally separated by commas. So the
comma-separated insertion patterns can be a pre-
processing target. Figure 4 shows examples. In the
first sentence, “if it ~ strong dollar” segment is an
inserted subordinate clause (INS_SB) and can be
extracted and handled independently. A segment
“Donald Taffner” from the second sentence is an
appositive of the preceding word “agent”. The
segment is extracted and translated separately, while
“Thames’s U.S. marketing agent is preparing to do
just that” is translated as one translation unit. In third
sentence, sentential modifier “in fact” is extracted
and the translation result is appended to the target
sentence. The identification and processing of the
insertion patterns can facilitate the parsing and
improve the quality of target sentence.
Figure 4: Examples of insertion patterns.
3.3 Post-processing Tasks
We explain post-processing tasks required by some
pre-processing tasks above described.
First, translation results of the translation units
split by non-word elements are combined using the
split element. The order of translation units must be
kept and split elements are inserted into the
combination positions. This post-processing locates
after the generation step in figure 1.
Second, enclosed parts by quotation marks or
parentheses are translated and then the results are
appended to the translation of the target words. In
post-processing, we search target words in the
resulting parse tree after the transfer steps in figure 1.
Third, we have to translate the combined date
and name-age words. This translation is based on the
translation patterns as in figure 5. This post-
processing is performed in transfer step in figure 1.
Fourth, segments with special patterns in section
3.2.3 are split according to their split information.
Also the patterns have information about how to
combine the parsing results of the split segments. In
this post-processing, we combine the parsing results
into one global sentence structure based on the
combination rules described in the patterns. That is,
"Next year, I may evaluate it a little closer," said
Stan Guest, an uninsured farmer.
 Q1, said Stan Guest, an uninsured farmer
 Stan Guest, an uninsured farmer, said, Q1.
"We continue to believe the position we've taken
is reasonable," a Morgan Stanley official said.
 Q1, a Morgan Stanley official said.
 A Morgan Stanley official said, Q1
A widening of the deficit, if it were combined
with a stubbornly strong dollar, would
exacerbate trade problems.
INS_SB: if it were combined with a stubbornly
strong dollar
Thames's U.S. marketing agent, Donald Taffner ,
is preparing to do just that.
INS_APP: Donald Taffner
Radio Free Europe, in fact, is in danger of
suffering from its success.
INS_MOD: in fact
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the post-processing is done after the partial parsing
in figure 1. Figure 6 shows examples of special
patterns consisting of 3 parts. The first part is a
sentence pattern, the second gives information how
to split the segment, and the third means how to
combine the parsing results. In the example, ‘+’
means combine two trees and ‘1_SUBJ_2’ means
the first tree’s subject is the second tree. In each
pattern, the first and second parts are for pre-
processing and the last part is for post-processing.
Figure 5: Examples of translation patterns for data and
name-age.
Figure 6: Examples of special patterns within a segment.
Fifth, during the sentence elements repositioning
in section 3.2.3, we identify the role of the segments.
In post-processing, we generate a global sentence
structure by combining partial parsing results as
directed by the information about role of segments.
Sixth, the parsing tree of the insertion segment is
added to the global sentence structure according to
the insertion types. For example, a tree for INS_SB
segment is added as subordinate clause, a tree for
INS_APP is added as appositive of the target word,
and a tree for INS_MOD is added as sentential
modifier. This post-processing can be performed
after constructing global sentence structure.
4 EXPERIMENTS
In this section, we show the usefulness of the
defined pre-processing tasks. For the purpose, we
search sentences with the defined pre-processing
problems. We use sentences from 4 domains in Penn
Treebank corpus: WSJ (Wall Street Journal), Brown,
ECTB (English-Chinese Tree Bank), and IBM. We
have 53,838 sentences from WSJ, 50,440 sentences
from Brown, 3,825 sentences from ECTB, and 4,404
sentences from IBM. We do not run the SmarTran
system, and only search the pre-processing problems
defined in section 3. From table 1 to table 3, we
summarize the defined pre-processing problems.
And the following three tables present the statistics
for the sentences with the problems.
Table 1: Pre-processing tasks before lexical analysis.
Task ID Description Target patterns
1 Separation by non-
word elements
semicolon, colon, bar
2 Separation by non-
word elements
‘’, “”, (), <>
3 Head leading list I, II, …, 1), 2), …,
4 Number + unit $, %, , dollar, cm, …
5 Composition words
6 Special patterns [ ~ so that ~ ], …
7 Date patterns [January NUMBER], …
Table 2: Pre-processing tasks after lexical analysis.
Task ID Description Target patterns
1 Human name
+ age
[PR-NOUN + NUMBER ]
2 Geographical
name
[PR-NOUN + , + PR-NOUN]
3 Head ADV,
AVP
42 adverbs, 4 adverbial phrases
4 Special
patterns
[ not only ~ but (also) ~ ], [ insist
-
like verbs that ~ VERB (bas
e
form) ~], …
5 Comma
rewriting
[ADJ(COMPR), ADJ(COMPR)],
[PR-NOUN(HYPHEN), PR
-
NOUN(HYPHEN)], …
Table 3: Pre-processing tasks after sentence segmentation.
Task ID Description Target patterns
1 Special
patterns
[ ~ so (such) ~ that ~ ], [ too
~
TO_INF ~ ], …
2 Say-like verbs say(said), tell(told), ask(asked),
explain(explained), …
Table 4: Statistics for sentences requiring pre-processing
before lexical analysis (%).
Task ID WSJ Brown ECTB IBM
1 6.7 11.1 4.7 0
2 20.1 19.7 13.3 8.3
3 0.4 0.4 0.2 0.2
4 13.1 1.0 1.5 0
5 5.2 5.6 9.1 2.7
6 0.6 1.1 0.7 0.5
7 5.7 1.7 4.7 0
51.8 40.6 34.2 11.7
[Month NUM1, NUM2]  [NUM2 년 Month 월
Num1 일]: January 1, 1998
[Month, NUM]  [NUM 년 Month 월]: January,
1998
[NUM, Month]  [Month 월 NUM 일]: 1,
January
[PRONOUN, NUM]  [NUM 살인
PRONOUN]
Antonio L. Savoca , 66 , was named
p
resident
1. [ ~ so A that B ], ( [~ so A] , [ and B ]) , +;
2. [ it BE-verb A ADJ that B ], ( [it BE-verb A
ADJ], [ B ] ), 1_SUBJ_2;
3. [ it BE-verb A that B ], ( [it BE-verb A],
[ B ] ), 1_SUBJ_2;
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261

Table 5: Statistics for sentences requiring pre-processing
after lexical analysis (%).
Task ID WSJ Brown ECTB IBM
1 0.6 0.4 0.5 0
2 7.1 3.2 8.2 0.4
3 9.7 8.8 3.3 2.5
4 0.2 0.4 0.5 0
5 0.2 0.1 0.1 0
17.7 13.0 12.7 3.0
Table 6: Statistics for sentences requiring pre-processing
after sentence segmentation (%).
Task ID WSJ Brown ECTB IBM
1 1.4 3.3 2.0 0.5
2 27.7 13.6 13.2 0.9
29.1 16.9 15.3 1.4
From the above three tables, we know that there
are many sentences which have the pre-processing
problems. The SmarTran can generate better
translation results using the pre-processing tasks.
Most sentences from WSJ can be benefited from the
pre-processing tasks. About 78% sentences from
Brown and about 60% from ECTB are also
benefited. Therefore, the proposed pre-processing
tasks are expected to improve the translation quality.
5 CONCLUSIONS
This paper presents required pre-processing tasks
and corresponding post-processing tasks in English-
Korean machine translation. The pre-processing has
purpose of solving differences between English and
Korean and facilitating the analysis of sentences
including non-word elements. Also, we classify the
tasks based on the time when the tasks should be
done. This classification augments the structure of
the existing EKMT system. For pre-processing
problems, we present solutions such as sentence split,
symbol or words deletion, word conversion,
combination of words, rewriting (words, phrases,
and comma), segment removal from the segment list
which is from sentence segmentation step, and
sentence elements repositioning.
Some of pre-processing solutions are already
developed, and others are being studied. We need a
representation method of patterns and other
information. Also, we must verify the solutions with
many examples. Some methods may cause side
effects, so we need solution to avoid them. Further,
we must test EKMT system with proposed pre- and
post-processing with many sentences and measure
how much the translation quality is improved. We
expect the pre-processing tasks will improve the
translation quality.
ACKNOWLEDGEMENTS
This research was supported by Basic Science
Research Program through the National Research
Foundation of Korea(KRF) funded by the Ministry
of Education, Science and Technology(2010-
0010815).
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