Table of contents

Appendices

F.1 Character Classes

A character class is an atom  R that identifies a set of characters  C(R). The set of strings L(R) denoted by a character class R contains one single-character string "c" for each character c in C(R).

A character class is either a character class escape or a character class expression.

A character class expression is a character group surrounded by [ and ] characters. For all character groups G, [G] is a valid character class expression, identifying the set of characters C([G]) = C(G).

A character group is either a positive character group, a negative character group, or a character class subtraction.

A positive character group consists of one or more character ranges or character class escapes, concatenated together. A positive character group identifies the set of characters containing all of the characters in all of the sets identified by its constituent ranges or escapes.

A negative character group is a positive character group preceded by the ^ character. For all positive character groups P, ^P is a valid negative character group, and C(^P) contains all XML characters that are not in C(P).

A character class subtraction is a character class expression subtracted from a positive character group or negative character group, using the - character.

For any positive character group or negative character group G, and any character class expression C, G-C is a valid character class subtraction, identifying the set of all characters in C(G) that are not also in C(C).

A character range R identifies a set of characters C(R) containing all XML characters with UCS code points in a specified range.

A single XML character is a character range that identifies the set of characters containing only itself. All XML characters are valid character ranges, except as follows:

• The [, ], and \ characters are not valid character ranges;

• The ^ character is only valid at the beginning of a positive character group if it is part of a negative character group; and

• The - character is a valid character range only at the beginning or end of a positive character group.

A character range may also be written in the form s-e, identifying the set that contains all XML characters with UCS code points greater than or equal to the code point of s, but not greater than the code point of e.

s-e is a valid character range iff:

• s is a single character escape, or an XML character;

• s is not \

• If s is the first character in a character class expression, then s is not ^

• e is a single character escape, or an XML character;

• e is not \ or [; and

• The code point of e is greater than or equal to the code point of s;

NOTE: 

The code point of a single character escape is the code point of the single character in the set of characters that it identifies.

Character Class Escapes[top]

A character class escape is a short sequence of characters that identifies predefined character class. The valid character class escapes are the single character escapes, the multi-character escapes, and the category escapes (including the block escapes).

A single character escape identifies a set containing a only one character -- usually because that character is difficult or impossible to write directly into a regular expression.

[UnicodeDB] specifies a number of possible values for the "General Category" property and provides mappings from code points to specific character properties. The set containing all characters that have property X, can be identified with a category escape \p{X}. The complement of this set is specified with the category escape \P{X}. ([\P{X}] = [^\p{X}]).

NOTE: 

[UnicodeDB] is subject to future revision. For example, the mapping from code points to character properties might be updated. All minimally conforming processors must support the character properties defined in the version of [UnicodeDB] that is current at the time this specification became a W3C Recommendation. However, implementors are encouraged to support the character properties defined in any future version.

The following table specifies the recognized values of the "General Category" property.

NOTE: 

The properties mentioned above exclude the Cs property. The Cs property identifies "surrogate" characters, which do not occur at the level of the "character abstraction" that XML instance documents operate on.

[UnicodeDB] groups code points into a number of blocks such as Basic Latin (i.e., ASCII), Latin-1 Supplement, Hangul Jamo, CJK Compatibility, etc. The set containing all characters that have block name X (with all white space stripped out), can be identified with a block escape \p{IsX}. The complement of this set is specified with the block escape \P{IsX}. ([\P{IsX}] = [^\p{IsX}]).

The following table specifies the recognized block names (for more information, see the "Blocks.txt" file in [UnicodeDB]).

NOTE: 

[UnicodeDB] is subject to future revision. For example, the grouping of code points into blocks might be updated. All minimally conforming processors must support the blocks defined in the version of [UnicodeDB] that is current at the time this specification became a W3C Recommendation. However, implementors are encouraged to support the blocks defined in any future version of the Unicode Standard.

For example, the block escape for identifying the ASCII characters is \p{IsBasicLatin}.

A multi-character escape provides a simple way to identify a commonly used set of characters:

NOTE: 

The regular expression language defined here does not attempt to provide a general solution to "regular expressions" over UCS character sequences. In particular, it does not easily provide for matching sequences of base characters and combining marks. The language is targeted at support of "Level 1" features as defined in [unicodeRegEx]. It is hoped that future versions of this specification will provide support for "Level 2" features.