UnicodeSet

Overview

A UnicodeSet is an object that represents a set of Unicode characters or character strings. The contents of that object can be specified either by patterns or by building them programmatically.

Here are a few examples of sets:

Pattern	Description
`[a-z]`	The lower case letters a through z
`[abc123]`	The six characters a,b,c,1,2 and 3
`[\p{Letter}]`	All characters with the Unicode General Category of Letter.

String Values

In addition to being a set of characters (of Unicode code points), a UnicodeSet may also contain string values. Conceptually, the UnicodeSet is always a set of strings, not a set of characters, although in many common use cases the strings are all of length one, which reduces to being a set of characters.

This concept can be confusing when first encountered, probably because similar set constructs from other environments (e.g., character classes in most regular expression implementations) can only contain characters.

Until ICU 68, it was not possible for a UnicodeSet to contain the empty string. In Java, an exception was thrown. In C++, the empty string was silently ignored.

Starting with ICU 69 ICU-13702 the empty string is supported as a set element; however, it is ignored in matching functions such as span(string).

UnicodeSet Patterns

Patterns are a series of characters bounded by square brackets that contain lists of characters and Unicode property sets. Lists are a sequence of characters that may have ranges indicated by a ‘-‘ between two characters, as in “a-z”. The sequence specifies the range of all characters from the left to the right, in Unicode order. For example, [a c d-f m] is equivalent to [a c d e f m]. Whitespace can be freely used for clarity as [a c d-f m] means the same as [acd-fm].

Unicode property sets are specified by a Unicode property, such as [:Letter:]. For a list of supported properties, see the Properties chapter. For details on the use of short vs. long property and property value names, see the end of this section. The syntax for specifying the property names is an extension of either POSIX or Perl syntax with the addition of “=value”. For example, you can match letters by using the POSIX syntax [:Letter:], or by using the Perl-style syntax \p{Letter}. The type can be omitted for the Category and Script properties, but is required for other properties.

The table below shows the two kinds of syntax: POSIX and Perl style. Also, the table shows the “Negative”, which is a property that excludes all characters of a given kind. For example, [:^Letter:] matches all characters that are not [:Letter:].

	Positive	Negative
POSIX-style Syntax	`[:type=value:]`	`[:^type=value:]`
Perl-style Syntax	`\p{type=value}`	`\P{type=value}`

These following low-level lists or properties then can be freely combined with the normal set operations (union, inverse, difference, and intersection):

	Example	Corresponding Method	Meaning
A B	`[[:letter:] [:number:]]`	`A.addAll(B)`	To union two sets A and B, simply concatenate them
A & B	`[[:letter:] & [a-z]]`	`A.retainAll(B)`	To intersect two sets A and B, use the ‘&’ operator.
A - B	`[[:letter:] - [a-z]]`	`A.removeAll(B)`	To take the set-difference of two sets A and B, use the ‘-‘ operator.
[^A]	`[^a-z]`	`A.complement(B)`	To invert a set A, place a ‘^’ immediately after the opening ‘[’. Note that the complement only affects code points, not string values. In any other location, the ‘^’ does not have a special meaning.

Precedence

The binary operators of union, intersection, and set-difference have equal precedence and bind left-to-right. Thus the following are equivalent:

[[:letter:] - [a-z] [:number:] & [\u0100-\u01FF]]
[[[[[:letter:] - [a-z]] [:number:]] & [\u0100-\u01FF]]

Another example is that the set [[ace][bdf\] - [abc][def]] is not the empty set, but instead the set [def]. That is because the syntax corresponds to the following UnicodeSet operations:

start with [ace]
addAll [bdf] – we now have [abcdef]
removeAll [abc] – we now have [def]
addAll [def] – no effect, we still have [def]

This only really matters where there are the difference and intersection operations, as the union operation is commutative. To make sure that the - is the main operator, add brackets to group the operations as desired, such as [[ace][bdf] - [[abc][def]]].

Another caveat with the ‘&’ and ‘-‘ operators is that they operate between sets. That is, they must be immediately preceded and immediately followed by a set. For example, the pattern [[:Lu:]-A] is illegal, since it is interpreted as the set [:Lu:] followed by the incomplete range -A. To specify the set of uppercase letters except for ‘A’, enclose the ‘A’ in a set: [[:Lu:]-[A]].

Examples

`[a]`	The set containing ‘a’
`[a-z]`	The set containing ‘a’ through ‘z’ and all letters in between, in Unicode order
`[^a-z]`	The set containing all characters but ‘a’ through ‘z’, that is, U+0000 through ‘a’-1 and ‘z’+1 through U+FFFF
`[[pat1][pat2]]`	The union of sets specified by pat1 and pat2
`[[pat1]& [pat2]]`	The intersection of sets specified by pat1 and pat2
`[[pat1]- [pat2]]`	The asymmetric difference of sets specified by pat1 and pat2
`[:Lu:]`	The set of characters belonging to the given Unicode category, as defined by `Character.getType()`; in this case, Unicode uppercase letters. The long form for this is `[:UppercaseLetter:]`.
`[:L:]`	The set of characters belonging to all Unicode categories starting with ‘L’, that is, `[[:Lu:][:Ll:][:Lt:][:Lm:][:Lo:]]`. The long form for this is `[:Letter:]`.

String Values in Sets

String values are enclosed in {curly brackets}.

Set expression	Description
`[abc{def}]`	A set containing four members, the single characters a, b and c, and the string “def”
`[{abc}{def}]`	A set containing two members, the string “abc” and the string “def”.
`[{a}{b}{c}]` `[abc]`	These two sets are equivalent. Each contains three items, the three individual characters a, b and c. A {string} containing a single character is equivalent to that same character specified in any other way.

Character Quoting and Escaping in Unicode Set Patterns

Single Quote

Two single quotes represents a single quote, either inside or outside single quotes.

Text within single quotes is not interpreted in any way (except for two adjacent single quotes). It is taken as literal text (special characters become non-special).

These quoting conventions for ICU UnicodeSets differ from those of regular expression character set expressions. In regular expressions, single quotes have no special meaning and are treated like any other literal character.

Backslash Escapes

Outside of single quotes, certain backslashed characters have special meaning:

`\uhhhh`	Exactly 4 hex digits; h in [0-9A-Fa-f]
`\Uhhhhhhhh`	Exactly 8 hex digits
`\xhh`	1-2 hex digits
`\ooo`	1-3 octal digits; o in [0-7]
`\a`	U+0007 (BELL)
`\b`	U+0008 (BACKSPACE)
`\t`	U+0009 (HORIZONTAL TAB)
`\n`	U+000A (LINE FEED)
`\v`	U+000B (VERTICAL TAB)
`\f`	U+000C (FORM FEED)
`\r`	U+000D (CARRIAGE RETURN)
`\\`	U+005C (BACKSLASH)

Anything else following a backslash is mapped to itself, except in an environment where it is defined to have some special meaning. For example, \\p{Lu} is the set of uppercase letters in UnicodeSet.

Any character formed as the result of a backslash escape loses any special meaning and is treated as a literal. In particular, note that \u and \U escapes create literal characters. (In contrast, the Java compiler treats Unicode escapes as just a way to represent arbitrary characters in an ASCII source file, and any resulting characters are not tagged as literals.)

Whitespace

Whitespace (as defined by our API) is ignored unless it is quoted or backslashed.

Note: The rules for quoting and white space handling are common to most ICU APIs that process rule or expression strings, including UnicodeSet, Transliteration and Break Iterators.

Note:ICU Regular Expression set expressions have a different (but similar) syntax, and a different set of recognized backslash escapes. [Sets] in ICU Regular Expressions follow the conventions from Perl and Java regular expressions rather than the pattern syntax from ICU UnicodeSet.

Using a UnicodeSet

For best performance, once the set contents is complete, freeze() the set to make it immutable and to speed up contains() and span() operations (for which it builds a small additional data structure).

The most basic operation is contains(code point) or, if relevant, contains(string).

For splitting and partitioning strings, it is simpler and faster to use span() and spanBack() rather than iterate over code points and calling contains(). In Java, there is also a class UnicodeSetSpanner for somewhat higher-level operations. See also the “Lookup” section of the Properties chapter.

Programmatically Building UnicodeSets

ICU users can programmatically build a UnicodeSet by adding or removing ranges of characters or by using the retain (intersection), remove (difference), and add (union) operations.

Property Values

The following property value variants are recognized:

Format	Description	Example
short	omits the type (used to prevent ambiguity and only allowed with the Category and Script properties)	Lu
medium	uses an abbreviated type and value	gc=Lu
long	uses a full type and value	General_Category=Uppercase_Letter

If the type or value is omitted, then the equals sign is also omitted. The short style is only used for Category and Script properties because these properties are very common and their omission is unambiguous.

In actual practice, you can mix type names and values that are omitted, abbreviated, or full. For example, if Category=Unassigned you could use what is in the table explicitly, \p{gc=Unassigned}, \p{Category=Cn}, or \p{Unassigned}.

When these are processed, case and whitespace are ignored so you may use them for clarity, if desired. For example, \p{Category = Uppercase Letter} or \p{Category = uppercase letter}.

For a list of supported properties, see the Properties chapter.

Getting UnicodeSet from Script

ICU provides the functionality of getting UnicodeSet from the script. Here is an example of generating a pattern from all the scripts that are associated to a Locale and then getting the UnicodeSet based on the generated pattern.

In C:

UErrorCode err = U_ZERO_ERROR;
const int32_t capacity = 10;
const char * shortname = NULL;
int32_t num, j;
int32_t strLength =4;
UChar32 c = 0x00003096 ;
UScriptCode script[10] = {USCRIPT_INVALID_CODE};
UScriptCode scriptcode = USCRIPT_INVALID_CODE;
num = uscript_getCode("ja",script,capacity, &err);
printf("%s %d \n" ,"Number of script code associated are :", num);
UnicodeString temp = UnicodeString("[", 1, US_INV);
UnicodeString pattern;
for(j=0;j<num;j++){
    shortname = uscript_getShortName(script[j]);
    UnicodeString str(shortname,strLength,US_INV);
    temp.append("[:");
    temp.append(str);
    temp.append(":]+");
}
pattern = temp.remove(temp.length()-1,1);
pattern.append("]");
UnicodeSet cnvSet(pattern, err);
printf("%d\n", cnvSet.size());
printf("%d\n", cnvSet.contains(c));

In Java:

ULocale ul = new ULocale("ja");
int script[] = UScript.getCode(ul);
String str ="[";
for(int i=0;i<script.length;i++){
    str = str + "[:"+UScript.getShortName(script[i])+":]+";
}
String pattern =str.substring(0, (str.length()-1));
pattern = pattern + "]";
System.out.println(pattern);
UnicodeSet ucs = new UnicodeSet(pattern);
System.out.println(ucs.size());
System.out.println(ucs.contains(0x00003096));