com.ibm.icu.text
Class RuleBasedTransliterator

java.lang.Object
  extended by com.ibm.icu.text.Transliterator
      extended by com.ibm.icu.text.RuleBasedTransliterator
All Implemented Interfaces:
StringTransform

Deprecated. This API is ICU internal only.

public class RuleBasedTransliterator
extends Transliterator

RuleBasedTransliterator is a transliterator that reads a set of rules in order to determine how to perform translations. Rule sets are stored in resource bundles indexed by name. Rules within a rule set are separated by semicolons (';'). To include a literal semicolon, prefix it with a backslash ('\'). Whitespace, as defined by UCharacterProperty.isRuleWhiteSpace(), is ignored. If the first non-blank character on a line is '#', the entire line is ignored as a comment.

Each set of rules consists of two groups, one forward, and one reverse. This is a convention that is not enforced; rules for one direction may be omitted, with the result that translations in that direction will not modify the source text. In addition, bidirectional forward-reverse rules may be specified for symmetrical transformations.

Rule syntax

Rule statements take one of the following forms:

$alefmadda=?;
Variable definition. The name on the left is assigned the text on the right. In this example, after this statement, instances of the left hand name, "$alefmadda", will be replaced by the Unicode character U+0622. Variable names must begin with a letter and consist only of letters, digits, and underscores. Case is significant. Duplicate names cause an exception to be thrown, that is, variables cannot be redefined. The right hand side may contain well-formed text of any length, including no text at all ("$empty=;"). The right hand side may contain embedded UnicodeSet patterns, for example, "$softvowel=[eiyEIY]".
 
ai>$alefmadda;
Forward translation rule. This rule states that the string on the left will be changed to the string on the right when performing forward transliteration.
 
ai<$alefmadda;
Reverse translation rule. This rule states that the string on the right will be changed to the string on the left when performing reverse transliteration.
ai<>$alefmadda;
Bidirectional translation rule. This rule states that the string on the right will be changed to the string on the left when performing forward transliteration, and vice versa when performing reverse transliteration.

Translation rules consist of a match pattern and an output string. The match pattern consists of literal characters, optionally preceded by context, and optionally followed by context. Context characters, like literal pattern characters, must be matched in the text being transliterated. However, unlike literal pattern characters, they are not replaced by the output text. For example, the pattern "abc{def}" indicates the characters "def" must be preceded by "abc" for a successful match. If there is a successful match, "def" will be replaced, but not "abc". The final '}' is optional, so "abc{def" is equivalent to "abc{def}". Another example is "{123}456" (or "123}456") in which the literal pattern "123" must be followed by "456".

The output string of a forward or reverse rule consists of characters to replace the literal pattern characters. If the output string contains the character '|', this is taken to indicate the location of the cursor after replacement. The cursor is the point in the text at which the next replacement, if any, will be applied. The cursor is usually placed within the replacement text; however, it can actually be placed into the precending or following context by using the special character '@'. Examples:

a {foo} z > | @ bar; # foo -> bar, move cursor before a
{foo} xyz > bar @@|; # foo -> bar, cursor between y and z

UnicodeSet

UnicodeSet patterns may appear anywhere that makes sense. They may appear in variable definitions. Contrariwise, UnicodeSet patterns may themselves contain variable references, such as "$a=[a-z];$not_a=[^$a]", or "$range=a-z;$ll=[$range]".

UnicodeSet patterns may also be embedded directly into rule strings. Thus, the following two rules are equivalent:

$vowel=[aeiou]; $vowel>'*'; # One way to do this
[aeiou]>'*';                # Another way

See UnicodeSet for more documentation and examples.

Segments

Segments of the input string can be matched and copied to the output string. This makes certain sets of rules simpler and more general, and makes reordering possible. For example:

([a-z]) > $1 $1;           # double lowercase letters
([:Lu:]) ([:Ll:]) > $2 $1; # reverse order of Lu-Ll pairs

The segment of the input string to be copied is delimited by "(" and ")". Up to nine segments may be defined. Segments may not overlap. In the output string, "$1" through "$9" represent the input string segments, in left-to-right order of definition.

Anchors

Patterns can be anchored to the beginning or the end of the text. This is done with the special characters '^' and '$'. For example:

^ a   > 'BEG_A';   # match 'a' at start of text
  a   > 'A';       # match other instances of 'a'
  z $ > 'END_Z';   # match 'z' at end of text
  z   > 'Z';       # match other instances of 'z'

It is also possible to match the beginning or the end of the text using a UnicodeSet. This is done by including a virtual anchor character '$' at the end of the set pattern. Although this is usually the match chafacter for the end anchor, the set will match either the beginning or the end of the text, depending on its placement. For example:

$x = [a-z$];   # match 'a' through 'z' OR anchor
$x 1    > 2;   # match '1' after a-z or at the start
   3 $x > 4;   # match '3' before a-z or at the end

Example

The following example rules illustrate many of the features of the rule language.

Rule 1. abc{def}>x|y
Rule 2. xyz>r
Rule 3. yz>q

Applying these rules to the string "adefabcdefz" yields the following results:

|adefabcdefz Initial state, no rules match. Advance cursor.
a|defabcdefz Still no match. Rule 1 does not match because the preceding context is not present.
ad|efabcdefz Still no match. Keep advancing until there is a match...
ade|fabcdefz ...
adef|abcdefz ...
adefa|bcdefz ...
adefab|cdefz ...
adefabc|defz Rule 1 matches; replace "def" with "xy" and back up the cursor to before the 'y'.
adefabcx|yz Although "xyz" is present, rule 2 does not match because the cursor is before the 'y', not before the 'x'. Rule 3 does match. Replace "yz" with "q".
adefabcxq| The cursor is at the end; transliteration is complete.

The order of rules is significant. If multiple rules may match at some point, the first matching rule is applied.

Forward and reverse rules may have an empty output string. Otherwise, an empty left or right hand side of any statement is a syntax error.

Single quotes are used to quote any character other than a digit or letter. To specify a single quote itself, inside or outside of quotes, use two single quotes in a row. For example, the rule "'>'>o''clock" changes the string ">" to the string "o'clock".

Notes

While a RuleBasedTransliterator is being built, it checks that the rules are added in proper order. For example, if the rule "a>x" is followed by the rule "ab>y", then the second rule will throw an exception. The reason is that the second rule can never be triggered, since the first rule always matches anything it matches. In other words, the first rule masks the second rule.

Copyright (c) IBM Corporation 1999-2000. All rights reserved.

Author:
Alan Liu
Status:
Internal. This API is ICU internal only.

Nested Class Summary
 
Nested classes/interfaces inherited from class com.ibm.icu.text.Transliterator
Transliterator.Factory, Transliterator.Position
 
Field Summary
 
Fields inherited from class com.ibm.icu.text.Transliterator
FORWARD, REVERSE
 
Method Summary
 UnicodeSet getTargetSet()
          Deprecated. This API is ICU internal only.
protected  UnicodeSet handleGetSourceSet()
          Deprecated. This API is ICU internal only.
protected  void handleTransliterate(Replaceable text, Transliterator.Position index, boolean incremental)
          Deprecated. This API is ICU internal only.
 String toRules(boolean escapeUnprintable)
          Deprecated. This API is ICU internal only.
 
Methods inherited from class com.ibm.icu.text.Transliterator
baseToRules, createFromRules, filteredTransliterate, finishTransliteration, getAvailableIDs, getAvailableSources, getAvailableTargets, getAvailableVariants, getDisplayName, getDisplayName, getDisplayName, getElements, getFilter, getID, getInstance, getInstance, getInverse, getMaximumContextLength, getSourceSet, registerAlias, registerClass, registerFactory, registerInstance, setFilter, setID, setMaximumContextLength, transform, transliterate, transliterate, transliterate, transliterate, transliterate, transliterate, unregister
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Method Detail

handleTransliterate

protected void handleTransliterate(Replaceable text,
                                   Transliterator.Position index,
                                   boolean incremental)
Deprecated. This API is ICU internal only.

Implements Transliterator.handleTransliterate(com.ibm.icu.text.Replaceable, com.ibm.icu.text.Transliterator.Position, boolean).

Specified by:
handleTransliterate in class Transliterator
Parameters:
text - the buffer holding transliterated and untransliterated text
index - the indices indicating the start, limit, context start, and context limit of the text.
incremental - if true, assume more text may be inserted at pos.limit and act accordingly. Otherwise, transliterate all text between pos.start and pos.limit and move pos.start up to pos.limit.
See Also:
Transliterator.transliterate(com.ibm.icu.text.Replaceable, int, int)
Status:
Internal. This API is ICU internal only.

toRules

public String toRules(boolean escapeUnprintable)
Deprecated. This API is ICU internal only.

Return a representation of this transliterator as source rules. These rules will produce an equivalent transliterator if used to construct a new transliterator.

Overrides:
toRules in class Transliterator
Parameters:
escapeUnprintable - if TRUE then convert unprintable character to their hex escape representations, \\uxxxx or \\Uxxxxxxxx. Unprintable characters are those other than U+000A, U+0020..U+007E.
Returns:
rules string
Status:
Internal. This API is ICU internal only.

handleGetSourceSet

protected UnicodeSet handleGetSourceSet()
Deprecated. This API is ICU internal only.

Return the set of all characters that may be modified by this Transliterator, ignoring the effect of our filter.

Overrides:
handleGetSourceSet in class Transliterator
Returns:
the set of characters that this transliterator may modify. The set may be modified, so subclasses should return a newly-created object.
See Also:
Transliterator.getSourceSet(), Transliterator.getTargetSet()
Status:
Internal. This API is ICU internal only.

getTargetSet

public UnicodeSet getTargetSet()
Deprecated. This API is ICU internal only.

Returns the set of all characters that may be generated as replacement text by this transliterator.

Overrides:
getTargetSet in class Transliterator
See Also:
Transliterator.getTargetSet()
Status:
Internal. This API is ICU internal only.


Copyright (c) 2009 IBM Corporation and others.