
Properties
Overview
Text processing requires that a program treat text appropriately. If text is exchanged between several systems, it is important for them to process the text consistently. This is done by assigning each character, or a range of characters, attributes or properties used for text processing, and by defining standard algorithms for at least the basic text operations.
Traditionally, such attributes and algorithms have not been well-defined for most character sets, and text processing had to rely on ad-hoc solutions. Over time, standards were created for querying properties of the system codepage. However, the set of these properties was limited. Their data was not coordinated among implementations, and standard algorithms were not available.
It is one of the strengths of Unicode that it not only defines a very large character set, but also assigns a comprehensive set of properties and usage notes to all characters. It defines standard algorithms for critical text processing, and the data is publicly provided and kept up-to-date. See http://www.unicode.org/ for more information.
Sample code is available in the ICU source code library at icu/source/samples/props/props.cpp . See also the source code for the Unicode browser demo application, which can be used online to browse Unicode characters with their properties.
Unicode Character Database properties in ICU APIs
The following table shows all Unicode Character Database properties (except for purely "extracted" ones and Unihan properties) and the corresponding ICU APIs. Most of the time, ICU4C provides functions in icu/source/common/unicode/uchar.h and ICU4J provides parallel functions in the com.ibm.icu.lang.UCharacter class. Properties of a single Unicode character are accessed by its 21-bit code point value (type: UChar32=int32_t in C/C++, int in Java). Most properties are also available via UnicodeSet APIs and patterns.
See the Unicode Character Database itself for comparison. PropertyAliases.txt lists all properties by name and type.
Most properties that use binary, integer, or enumerated values are available via functions u_hasBinaryProperty and u_getIntPropertyValue which take UProperty enum constants to select the property. (ICU4J UCharacter member functions do not have the "u_" prefix.) The constant names include the long property name according to PropertyAliases.txt, e.g., UCHAR_LINE_BREAK. Corresponding property value enum constant names often contain the short property name and the long value name, e.g., U_LB_LINE_FEED. For enumeration/integer type properties, the enumeration result type is also listed here.
Some UnicodeSet APIs use the same UProperty constants. Other UnicodeSet APIs and UnicodeSet and regular expression patterns use the long or short property aliases and property value aliases (see PropertyAliases.txt and PropertyValueAliases.txt).
There is one pseudo-property, UCHAR_GENERAL_CATEGORY_MASK for which the APIs do not use a single value but a bit-set (a mask) of zero or more values, with each bit corresponding to one UCHAR_GENERAL_CATEGORY value. This allows ICU to represent property value aliases for multiple general categories, like "Letters" (which stands for "Uppercase Letters", "Lowercase Letters", etc.). In other words, there are two ICU properties for the same Unicode property, one delivering single values (for per-code point lookup) and the other delivering sets of values (for use with value aliases and UnicodeSet).
UCD Name (see PropertyAliases.txt) | Type | ICU4C uchar.h ICU4J UCharacter | UCD File (.txt) | |
---|---|---|---|---|
Age | Unicode version | (U) | C: u_charAge fills in UVersionInfo Java: getAge returns a VersionInfo reference | DerivedAge |
Alphabetic | binary | (U) | u_isUAlphabetic, UCHAR_ALPHABETIC | DerivedCoreProperties |
ASCII_Hex_Digit | binary | (U) | UCHAR_ASCII_HEX_DIGIT | PropList |
Bidi_Class | enum UCharDirection | (U) | u_charDirection, UCHAR_BIDI_CLASS | UnicodeData |
Bidi_Control | binary | (U) | UCHAR_BIDI_CONTROL | PropList |
Bidi_Mirrored | binary | (U) | u_isMirrored, UCHAR_BIDI_MIRRORED | UnicodeData |
Bidi_Mirroring_Glyph | code point | u_charMirror | BidiMirroring | |
Block | enum UBlockCode (growing) | (U) | ublock_getCode, UCHAR_BLOCK | Blocks |
Canonical_Combining_Class | 0..255 | (U) | u_getCombiningClass, UCHAR_CANONICAL_COMBINING_CLASS | UnicodeData |
Case_Folding | Unicode string | u_strFoldCase (ustring.h) | CaseFolding | |
Composition_Exclusion | binary | (c) | contributes to Full_Composition_Exclusion | CompositionExclusions |
Dash | binary | (U) | UCHAR_DASH | PropList |
Decomposition_Mapping | Unicode string | available via normalization API | UnicodeData | |
Decomposition_Type | enum UDecompositionType | (U) | UCHAR_DECOMPOSITION_TYPE | UnicodeData |
Default_Ignorable_Code_Point | binary | (U) | UCHAR_DEFAULT_IGNORABLE_CODE_POINT | DerivedCoreProperties |
Deprecated | binary | (U) | UCHAR_DEPRECATED | PropList |
Diacritic | binary | (U) | UCHAR_DIACRITIC | PropList |
East_Asian_Width | enum UEastAsianWidth | (U) | UCHAR_EAST_ASIAN_WIDTH | EastAsianWidth |
Expands_On_NF* | binary | available via normalization API (unorm.h) | DerivedNormalizationProps | |
Extender | binary | (U) | UCHAR_EXTENDER | PropList |
FC_NFKC_Closure | Unicode string | u_getFC_NFKC_Closure | DerivedNormalizationProps | |
Full_Composition_Exclusion | binary | (U) | UCHAR_FULL_COMPOSITION_EXCLUSION | DerivedNormalizationProps |
General_Category | enum (<= 32 values) | (U) | u_charType, UCHAR_GENERAL_CATEGORY, UCHAR_GENERAL_CATEGORY_MASK, UCharCategory | UnicodeData |
Grapheme_Base | binary | (U) | UCHAR_GRAPHEME_BASE | DerivedCoreProperties |
Grapheme_Extend | binary | (U) | UCHAR_GRAPHEME_EXTEND | DerivedCoreProperties |
Grapheme_Link | binary | (U) | UCHAR_GRAPHEME_LINK | DerivedCoreProperties |
Hex_Digit | binary | (U) | UCHAR_HEX_DIGIT | PropList |
Hyphen | binary | (U) | UCHAR_HYPHEN | PropList |
ID_Continue | binary | (U) | UCHAR_ID_CONTINUE | DerivedCoreProperties |
ID_Start | binary | (U) | UCHAR_ID_START | DerivedCoreProperties |
Ideographic | binary | (U) | UCHAR_IDEOGRAPHIC | PropList |
IDS_Binary_Operator | binary | (U) | UCHAR_IDS_BINARY_OPERATOR | PropList |
IDS_Triary_Operator | binary | (U) | UCHAR_IDS_TRINARY_OPERATOR | PropList |
ISO_Comment | ASCII string | u_getISOComment | UnicodeData | |
Join_Control | binary | (U) | UCHAR_JOIN_CONTROL | PropList |
Joining_Group | enum UJoiningGroup | (U) | UCHAR_JOINING_GROUP | ArabicShaping |
Joining_Type | enum UJoiningType | (U) | UCHAR_JOINING_TYPE | ArabicShaping |
Line_Break | enum ULineBreak | (U) | UCHAR_LINE_BREAK | LineBreak |
Logical_Order_Exception | binary | (U) | UCHAR_LOGICAL_ORDER_EXCEPTION | PropList |
Lowercase | binary | (U) | u_isULowercase, UCHAR_LOWERCASE | DerivedCoreProperties |
Lowercase_Mapping | Unicode string + conditions | available via u_strToLower (ustring.h) | UnicodeData + SpecialCasing | |
Math | binary | (U) | UCHAR_MATH | DerivedCoreProperties |
Name | ASCII string | (U) | u_charName(U_UNICODE_CHAR_NAME or U_EXTENDED_CHAR_NAME) | UnicodeData |
NF*_QuickCheck | no/maybe/yes | available via unorm_quickCheck (unorm.h) | DerivedNormalizationProps | |
Noncharacter_Code_Point | binary | (U) | UCHAR_NONCHARACTER_CODE_POINT, U_IS_UNICODE_NONCHAR (utf.h) | PropList |
Numeric_Type | enum UNumericType | (U) | UCHAR_NUMERIC_TYPE | UnicodeData |
Numeric_Value | double | (U) | u_getNumericValue Java/UnicodeSet: only non-negative integers, no fractions | UnicodeData |
Other_Alphabetic | binary | (c) | contributes to Alphabetic | PropList |
Other_Default_Ignorable_Code_Point | binary | (c) | contributes to Default_Ignorable_Code_Point | PropList |
Other_Grapheme_Extend | binary | (c) | contributes to Grapheme_Extend | PropList |
Other_Lowercase | binary | (c) | contributes to Lowercase | PropList |
Other_Math | binary | (c) | contributes to Math | PropList |
Other_Uppercase | binary | (c) | contributes to Uppercase | PropList |
Quotation_Mark | binary | (U) | UCHAR_QUOTATION_MARK | PropList |
Radical | binary | (U) | UCHAR_RADICAL | PropList |
Script | enum UScriptCode (growing) | (U) | uscript_getCode (uscript.h), UCHAR_SCRIPT | Scripts |
Simple_Case_Folding | code point | u_foldCase | CaseFolding | |
Simple_Lowercase_ Mapping | code point | u_tolower | UnicodeData | |
Simple_Titlecase_ Mapping | code point | u_totitle | UnicodeData | |
Simple_Uppercase_ Mapping | code point | u_toupper | UnicodeData | |
Soft_Dotted | binary | (U) | UCHAR_SOFT_DOTTED | PropList |
Special_Case_Condition | conditions | available via u_strToLower etc. (ustring.h) | SpecialCasing | |
Terminal_Punctuation | binary | (U) | UCHAR_TERMINAL_PUNCTUATION | PropList |
Titlecase_Mapping | Unicode string + conditions | u_strToTitle (ustring.h) | UnicodeData + SpecialCasing | |
Unicode_1_Name | ASCII string | (U) | u_charName(U_UNICODE_10_CHAR_NAME or U_EXTENDED_CHAR_NAME) | UnicodeData |
Unified_Ideograph | binary | (U) | UCHAR_UNIFIED_IDEOGRAPH | PropList |
Uppercase | binary | (U) | u_isUUppercase, UCHAR_UPPERCASE | DerivedCoreProperties |
Uppercase_Mapping | Unicode string + conditions | u_strToUpper (ustring.h) | UnicodeData + SpecialCasing | |
White_Space | binary | (U) | u_isUWhiteSpace, UCHAR_WHITE_SPACE | PropList |
XID_Continue | binary | (U) | UCHAR_XID_CONTINUE | DerivedCoreProperties |
XID_Start | binary | (U) | UCHAR_XID_START | DerivedCoreProperties |
Notes:
(c) - This property only contributes to "real" properties (mostly "Other_..." properties), so there is no direct support for this property in ICU.
(U) - This property is available via the UnicodeSet APIs and patterns. Any property available in UnicodeSet is also available in regular expressions. Properties which are not available in UnicodeSet are generally those that are not available through a UProperty selector.
Customization
ICU does not provide the means to modify properties at runtime. The properties are provided exactly as specified by a recent version of the Unicode Standard (as published in the Character Database ). However, if an application requires custom properties (for example, for Private Use characters), then it is possible to change or add them at build-time. This is done by modifying the Character Database files copied into the ICU source tree at icu/source/data/unidata. For the most common properties, the file to modify is UnicodeData.txt.
To add a character to such a file, a line must be inserted into the file with the format used in that file (see the online documentation on the Unicode site for more information). These files are processed by ICU tools at build time. For example, the genprops tool reads several of the files and writes the binary file uprops.dat, which is then packaged into the common ICU data file. It is important for the operation of those tools that the Unicode character code points of the entries are in ascending order (gaps are allowed). Any available Unicode code point (0 to 10ffff16) can be used. Code point values should be written with either 4, 5, or 6 hex digits. The minimum number of digits possible should be used (but no fewer than 4). Note that the Unicode Standard specifies that the 32 code point U+fdd0..U+fdef and the 34 code points U+...fffe and U+...ffff are not characters, therefore they should not be added to any of the character database files.
After modifying one of these files, the ICU data needs to be rebuilt. The makefiles should detect the modifications and run the necessary tools automatically.
Copyright (c) 2000 - 2005 IBM and Others - PDF Version - Feedback: http://icu.sourceforge.net/contacts.html
User Guide for ICU v3.4 Generated 2005-07-27.