Class RuleBasedTransliterator

  • All Implemented Interfaces:
    StringTransform, Transform<java.lang.String,​java.lang.String>

    @Deprecated
    public class RuleBasedTransliterator
    extends Transliterator
    Deprecated.
    This API is ICU internal only.
    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 ('\'). Unicode Pattern_White_Space 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:

    $alefmadda=آ;
    Variable definition. The name on the left is assigned the text on the right. In this example, after this statement, instances of the left hand name, "$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.

    Author:
    Alan Liu
    • Method Detail

      • toRules

        @Deprecated
        public java.lang.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
      • addSourceTargetSet

        @Deprecated
        public void addSourceTargetSet​(UnicodeSet filter,
                                       UnicodeSet sourceSet,
                                       UnicodeSet targetSet)
        Deprecated.
        This API is ICU internal only.
        Description copied from class: Transliterator
        Returns the set of all characters that may be generated as replacement text by this transliterator, filtered by BOTH the input filter, and the current getFilter().

        SHOULD BE OVERRIDEN BY SUBCLASSES. It is probably an error for any transliterator to NOT override this, but we can't force them to for backwards compatibility.

        Other methods vector through this.

        When gathering the information on source and target, the compound transliterator makes things complicated. For example, suppose we have:

         Global FILTER = [ax]
         a > b;
         :: NULL;
         b > c;
         x > d;
         
        While the filter just allows a and x, b is an intermediate result, which could produce c. So the source and target sets cannot be gathered independently. What we have to do is filter the sources for the first transliterator according to the global filter, intersect that transliterator's filter. Based on that we get the target. The next transliterator gets as a global filter (global + last target). And so on.

        There is another complication:

         Global FILTER = [ax]
         a >|b;
         b >c;
         
        Even though b would be filtered from the input, whenever we have a backup, it could be part of the input. So ideally we will change the global filter as we go.
        Overrides:
        addSourceTargetSet in class Transliterator
        targetSet - TODO
        See Also:
        Transliterator.getTargetSet()
      • safeClone

        @Deprecated
        public Transliterator safeClone()
        Deprecated.
        This API is ICU internal only.
        Temporary hack for registry problem. Needs to be replaced by better architecture.