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RuleBasedTransliterator Class Reference

#include <rbt.h>

Inheritance diagram for RuleBasedTransliterator:

Transliterator UObject UMemory

List of all members.

Detailed Description

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 Character.isWhitespace(), 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:

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]".  
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.
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.

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 '</code>'. Examples:


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


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 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.


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


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".


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.

Alan Liu

For internal use only.

Use transliterator factory methods instead since this class will be removed in that release.

Definition at line 289 of file rbt.h.

Public Types

typedef Transliterator
*(U_EXPORT2 * 
Factory )(const UnicodeString &ID, Token context)

Public Member Functions

void adoptFilter (UnicodeFilter *adoptedFilter)
virtual Transliteratorclone (void) const
int32_t countElements () const
TransliteratorcreateInverse (UErrorCode &status) const
virtual void filteredTransliterate (Replaceable &text, UTransPosition &index, UBool incremental) const
virtual void finishTransliteration (Replaceable &text, UTransPosition &index) const
virtual UClassID getDynamicClassID (void) const
const TransliteratorgetElement (int32_t index, UErrorCode &ec) const
const UnicodeFiltergetFilter (void) const
virtual const UnicodeStringgetID (void) const
int32_t getMaximumContextLength (void) const
UnicodeSetgetSourceSet (UnicodeSet &result) const
virtual UnicodeSetgetTargetSet (UnicodeSet &result) const
UnicodeFilterorphanFilter (void)
 RuleBasedTransliterator (const RuleBasedTransliterator &)
 RuleBasedTransliterator (const UnicodeString &id, const UnicodeString &rules, UTransDirection direction, UnicodeFilter *adoptedFilter, UParseError &parseError, UErrorCode &status)
virtual UnicodeStringtoRules (UnicodeString &result, UBool escapeUnprintable) const
virtual void transliterate (Replaceable &text, UTransPosition &index, UErrorCode &status) const
virtual void transliterate (Replaceable &text, UTransPosition &index, UChar32 insertion, UErrorCode &status) const
virtual void transliterate (Replaceable &text, UTransPosition &index, const UnicodeString &insertion, UErrorCode &status) const
virtual void transliterate (Replaceable &text) const
virtual int32_t transliterate (Replaceable &text, int32_t start, int32_t limit) const
virtual ~RuleBasedTransliterator ()

Static Public Member Functions

static int32_t U_EXPORT2 countAvailableIDs (void)
static int32_t U_EXPORT2 countAvailableSources (void)
static int32_t U_EXPORT2 countAvailableTargets (const UnicodeString &source)
static int32_t U_EXPORT2 countAvailableVariants (const UnicodeString &source, const UnicodeString &target)
static Transliterator *U_EXPORT2 createFromRules (const UnicodeString &ID, const UnicodeString &rules, UTransDirection dir, UParseError &parseError, UErrorCode &status)
static Transliterator *U_EXPORT2 createInstance (const UnicodeString &ID, UTransDirection dir, UErrorCode &status)
static Transliterator *U_EXPORT2 createInstance (const UnicodeString &ID, UTransDirection dir, UParseError &parseError, UErrorCode &status)
static const UnicodeString
getAvailableID (int32_t index)
static StringEnumeration *U_EXPORT2 getAvailableIDs (UErrorCode &ec)
static UnicodeString &U_EXPORT2 getAvailableSource (int32_t index, UnicodeString &result)
static UnicodeString &U_EXPORT2 getAvailableTarget (int32_t index, const UnicodeString &source, UnicodeString &result)
static UnicodeString &U_EXPORT2 getAvailableVariant (int32_t index, const UnicodeString &source, const UnicodeString &target, UnicodeString &result)
static UnicodeString &U_EXPORT2 getDisplayName (const UnicodeString &ID, const Locale &inLocale, UnicodeString &result)
static UnicodeString &U_EXPORT2 getDisplayName (const UnicodeString &ID, UnicodeString &result)
static U_I18N_API UClassID
getStaticClassID (void)
static Token integerToken (int32_t)
static void U_EXPORT2 operator delete (void *, void *) U_NO_THROW
static void U_EXPORT2 operator delete (void *p) U_NO_THROW
static void U_EXPORT2 operator delete[] (void *p) U_NO_THROW
static void *U_EXPORT2 operator new (size_t, void *ptr) U_NO_THROW
static void *U_EXPORT2 operator new (size_t size) U_NO_THROW
static void *U_EXPORT2 operator new[] (size_t size) U_NO_THROW
static Token pointerToken (void *)
static void U_EXPORT2 registerAlias (const UnicodeString &aliasID, const UnicodeString &realID)
static void U_EXPORT2 registerFactory (const UnicodeString &id, Factory factory, Token context)
static void U_EXPORT2 registerInstance (Transliterator *adoptedObj)
static void U_EXPORT2 unregister (const UnicodeString &ID)

Protected Member Functions

virtual void handleGetSourceSet (UnicodeSet &result) const
virtual void handleTransliterate (Replaceable &text, UTransPosition &offsets, UBool isIncremental) const
void setID (const UnicodeString &id)
void setMaximumContextLength (int32_t maxContextLength)

Static Protected Member Functions

static int32_t _countAvailableSources (void)
static int32_t _countAvailableTargets (const UnicodeString &source)
static int32_t _countAvailableVariants (const UnicodeString &source, const UnicodeString &target)
static UnicodeString_getAvailableSource (int32_t index, UnicodeString &result)
static UnicodeString_getAvailableTarget (int32_t index, const UnicodeString &source, UnicodeString &result)
static UnicodeString_getAvailableVariant (int32_t index, const UnicodeString &source, const UnicodeString &target, UnicodeString &result)
static void _registerAlias (const UnicodeString &aliasID, const UnicodeString &realID)
static void _registerFactory (const UnicodeString &id, Factory factory, Token context)
static void _registerInstance (Transliterator *adoptedObj)
static void _registerSpecialInverse (const UnicodeString &target, const UnicodeString &inverseTarget, UBool bidirectional)
static TransliteratorcreateBasicInstance (const UnicodeString &id, const UnicodeString *canon)

Private Member Functions

void _construct (const UnicodeString &rules, UTransDirection direction, UParseError &parseError, UErrorCode &status)
 RuleBasedTransliterator (const UnicodeString &id, TransliterationRuleData *data, UBool isDataAdopted)
 RuleBasedTransliterator (const UnicodeString &id, const TransliterationRuleData *theData, UnicodeFilter *adoptedFilter=0)

Private Attributes

UBool isDataOwned


class Transliterator
class TransliteratorAlias
class TransliteratorIDParser
class TransliteratorParser
class TransliteratorRegistry

The documentation for this class was generated from the following files:

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