Character Classes 

A character class represents a set of characters that can match an input string. Combine literal characters, escape characters, and character classes to form a regular expression pattern.

Character classes define sets of characters. Some character classes are equivalent to one or more Unicode general category values or Unicode blocks. A Unicode general category defines the broad classification of a character; that is, whether the character is a type of letter, decimal digit, separator, mathematical symbol, punctuation, and so on. For example, the Lu general category represents "Letter, Uppercase" and the Sm category represents "Symbol, Math". For more information, see Supported Unicode General Categories.

A Unicode block is a named range of Unicode code points. The .NET Framework provides a set of named blocks derived from the Unicode block names. For example, the .NET Framework provides the IsBasicLatin named block, which corresponds to the Basic Latin Unicode block and contains characters ranging from U+0000 through U+007F. For more information, see Supported Named Blocks.

The .NET Framework supports character class subtraction expressions, which enables you to define a set of characters as the result of excluding one character class from another character class. For more information, see Character Class Subtraction.

Character Class Syntax

The following table summarizes the character classes and their syntax.

Supported Unicode General Categories

Unicode defines the general categories and descriptions listed in the following table. For more information, see the "UCD File Format" and "General Category Values" subtopics at the Unicode Character Database.

The .NET Framework provides additional categories that represent a set of Unicode character categories, as shown in the following table.

Supported Named Blocks

The .NET Framework provides the named blocks listed in the following table. The set of supported named blocks is based on Unicode 4.0 and Perl 5.6.

Character Class Subtraction

A character class defines a set of characters. Character class subtraction yields a set of characters that is the result of excluding the characters in one character class from another character class.

A character class subtraction expression has the following form:

[base_group-[excluded_group]]

The square brackets ([]) and hyphen (-) are mandatory. The base_group is a positive or negative character group as described in the Character Class Syntax table. The excluded_group component is another positive or negative character group, or another character class subtraction expression (that is, you can nest character class subtraction expressions).

For example, suppose you have a base group that consists of the character range from 'a' through 'z'. To define the set of characters that consists of the base group except for the character 'm', use [a-z-[m]]. To define the set of characters that consists of the base group except for the set of characters 'd', 'j', and 'p', use [a-z-[djp]]. To define the set of characters that consists of the base group except for the character range from 'm' through 'p', use [a-z-[m-p]].

Consider the nested character class subtraction expression, [a-z-[d-w-[m-o]]]. The expression is evaluated from the innermost character range outward. First, the character range from 'm' through 'o' is subtracted from the character range 'd' through 'w', which yields the set of characters from 'd' through 'l' and 'p' through 'w'. That set is then subtracted from the character range from 'a' through 'z', which yields the set of characters, [abcmnoxyz].

You can use any character class with character class subtraction. To define the set of characters that consists of all Unicode characters from \u0000 through \uFFFF except white-space characters (\s), the characters in the punctuation general category (\p{P}), the characters in the IsGreek named block (\p{IsGreek}), and the Unicode NEXT LINE control character (\x85), use [\u0000-\uFFFF-[\s\p{P}\p{IsGreek}\x85]].

Choose character classes for a character class subtraction expression that will yield useful results. Avoid an expression that yields an empty set of characters, which cannot match anything, or an expression that is equivalent to the original base group. For example, the empty set is the result of the expression [\p{IsBasicLatin}-[\x00-\x7F]], which subtracts all characters from the IsBasicLatin general category. Similarly, the original base group is the result of the expression [a-z-[0-9]]. This is because the base group, which is the character range of letters from 'a' through 'z', does not contain any characters in the excluded group, which is the character range of decimal digits from '0' through '9'.

Note that XML Schema Regular Expressions has similar support for character class subtraction.

See Also

Reference

GetUnicodeCategory
Regular Expression Options

Other Resources

Regular Expression Language Elements