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6.4. More Atoms

By this point, we have seen the simplest atoms that can be used in a pattern: "1," "5," and "\." are atoms that exactly match a character. The other atoms that can be used in patterns are special characters, a wildcard that matches any character, or predefined and user-defined character classes.

6.4.3. Character Classes

W3C XML Schema has adopted the "classical" Perl and Unicode character classes (but not the POSIX-style character classes also available in Perl).

6.4.3.1. Classical Perl character classes

W3C XML Schema supports the classical Perl character classes plus a couple of additions to match XML-specific productions. Each of these classes are designated by a single letter; the classes designated by the upper- and lowercase versions of the same letter are complementary:

\s
Spaces. Matches the XML whitespaces (space #x20, tabulation #x09, line feed #x0A, and carriage return #x0D).

\S
Characters that are not spaces.

\d
Digits ("0" to "9" but also digits in other alphabets).

\D
Characters that are not digits.

\w
Extended "word" characters (any Unicode character not defined as "punctuation", "separator," and "other"). This conforms to the Perl definition, assuming UTF8 support has been switched on.

\W
Nonword characters.

\i
XML 1.0 initial name characters (i.e., all the "letters" plus "-"). This is a W3C XML Schema extension over Perl regular expressions.

\I
Characters that may not be used as a XML initial name character.

\c
XML 1.0 name characters (initial name characters, digits, ".", ":", "-", and the characters defined by Unicode as "combining" or "extender"). This is a W3C XML Schema extension to Perl regular expressions.

\C
Characters that may not be used in a XML 1.0 name.

These character classes may be used with an optional quantifier like any other atom. The last pattern that we saw:

<xs:pattern value=".*0"/>

constrains the lexical space to be a string of characters ending with a zero. Knowing that the base type is a xs:integer, this is good enough for our purposes, but if the base type had been a xs:decimal (or xs:string), we could be more restrictive and write:

<xs:pattern value="-?\d*0"/>

This checks that the characters before the trailing zero are digits with an optional leading - (we will see later on in Section 6.5.2.2, "Fixed format" how to specify an optional leading - or +).

6.4.3.2. Unicode character classes

Patterns support character classes matching both Unicode categories and blocks. Categories and blocks are two complementary classification systems: categories classify the characters by their usage independently to their localization (letters, uppercase, digit, punctuation, etc.), while blocks classify characters by their localization independently of their usage (Latin, Arabic, Hebrew, Tibetan, and even Gothic or musical symbols).

The syntax \p{Name} is similar for blocks and categories; the prefix Is is added to the name of categories to make the distinction. The syntax \P{Name} is also available to select the characters that do not match a block or category. A list of Unicode blocks and categories is given in the specification. Table 6-2 shows the Unicode character classes and Table 6-3 shows the Unicode character blocks.

Table 6-2. Unicode character classes

Unicode Character Class

Includes

C

Other characters (non-letters, non symbols, non-numbers, non-separators)

Cc

Control characters

Cf

Format characters

Cn

Unassigned code points

Co

Private use characters

L

Letters

Ll

Lowercase letters

Lm

Modifier letters

Lo

Other letters

Lt

Titlecase letters

Lu

Uppercase letters

M

All Marks

Mc

Spacing combining marks

Me

Enclosing marks

Mn

Non-spacing marks

N

Numbers

Nd

Decimal digits

Nl

Number letters

No

Other numbers

P

Punctuation

Pc

Connector punctuation

Pd

Dashes

Pe

Closing punctuation

Pf

Final quotes (may behave like Ps or Pe)

Pi

Initial quotes (may behave like Ps or Pe)

Po

Other forms of punctuation

Ps

Opening punctuation

S

Symbols

Sc

Currency symbols

Sk

Modifier symbols

Sm

Mathematical symbols

So

Other symbols

Z

Separators

Zl

Line breaks

Zp

Paragraph breaks

Zs

Spaces

Table 6-3. Unicode character blocks

AlphabeticPresentationForms

Arabic

ArabicPresentationForms-A

ArabicPresentationForms-B

Armenian

Arrows

BasicLatin

Bengali

BlockElements

Bopomofo

BopomofoExtended

BoxDrawing

BraillePatterns

ByzantineMusicalSymbols

Cherokee

CJKCompatibility

CJKCompatibilityForms

CJKCompatibilityIdeographs

CJKCompatibilityIdeographsSupplement

CJKRadicalsSupplement

CJKSymbolsandPunctuation

CJKUnifiedIdeographs

CJKUnifiedIdeographsExtensionA

CJKUnifiedIdeographsExtensionB

CombiningDiacriticalMarks

CombiningHalfMarks

CombiningMarksforSymbols

ControlPictures

CurrencySymbols

Cyrillic

Deseret

Devanagari

Dingbats

EnclosedAlphanumerics

EnclosedCJKLettersandMonths

Ethiopic

GeneralPunctuation

GeometricShapes

Georgian

Gothic

Greek

GreekExtended

Gujarati

Gurmukhi

HalfwidthandFullwidthForms

HangulCompatibilityJamo

HangulJamo

HangulSyllables

Hebrew

HighPrivateUseSurrogates

HighSurrogates

Hiragana

IdeographicDescriptionCharacters

IPAExtensions

Kanbun

KangxiRadicals

Kannada

Katakana

Khmer

Lao

Latin-1Supplement

LatinExtended-A

LatinExtendedAdditional

LatinExtended-B

LetterlikeSymbols

LowSurrogates

Malayalam

MathematicalAlphanumericSymbols

MathematicalOperators

MiscellaneousSymbols

MiscellaneousTechnical

Mongolian

MusicalSymbols

Myanmar

NumberForms

Ogham

OldItalic

OpticalCharacterRecognition

Oriya

PrivateUse

PrivateUse

PrivateUse

Runic

Sinhala

SmallFormVariants

SpacingModifierLetters

Specials

Specials

SuperscriptsandSubscripts

Syriac

Tags

Tamil

Telugu

Thaana

Thai

Tibetan

UnifiedCanadianAboriginalSyllabics

YiRadicals

YiSyllables

We don't yet know how to specify intersections between a block and a category in a single pattern, or how to specify that a datatype must be composed of only basic Latin letters. So, to "cross" these classifications and define the intersection of the block L (all the letters) and the category BasicLatin (ASCII characters below #x7F), we can perform two successive restrictions:

<xs:simpleType name="BasicLatinLetters">
  <xs:restriction>
    <xs:simpleType>
      <xs:restriction base="xs:token">
        <xs:pattern value="\p{IsBasicLatin}*"/>
      </xs:restriction>
    </xs:simpleType>
    <xs:pattern value="\p{L}*"/>
  </xs:restriction>
</xs:simpleType>

6.4.3.3. User-defined character classes

These classes are lists of characters between square brackets that accept - signs to define ranges and a leading ^ to negate the whole list--for instance:

[azertyuiop]

to define the list of letters on the first row of a French keyboard,

[a-z]

to specify all the characters between "a" and "z",

[^a-z]

for all the characters that are not between "a" and "z," but also

[-^\\]

to define the characters "-," "^," and "\," or

[-+]

to specify a decimal sign.

These examples are enough to see that what's between these square brackets follows a specific syntax and semantic. Like the regular expression's main syntax, we have a list of atoms, but instead of matching each atom against a character of the instance string, we define a logical space. Between the atoms and the character class is the set of characters matching any of the atoms found between the brackets.

We see also two special characters that have a different meaning depending on their location! The character -, which is a range delimiter when it is between a and z, is a normal character when it is just after the opening bracket or just before the closing bracket ([+-] and [-+] are, therefore, both legal). On the contrary, ^, which is a negator when it appears at the beginning of a class, loses this special meaning to become a normal character later in the class definition.

We also notice that characters may or must be escaped: "\\" is used to match the character "\". In fact, in a class definition, all the escape sequences that we have seen as atoms can be used. Even though some of the special characters lose their special meaning inside square brackets, they can always be escaped. So, the following:

[-^\\]

can also be written as:

[\-\^\\]

or as:

[\^\\-]

since the location of the characters doesn't matter any longer when they are escaped.

Within square brackets, the character "\" also keeps its meaning of a reference to a Perl or Unicode class. The following:

[\d\p{Lu}]

is a set of decimal digits (Perl class \d) and uppercase letters (Unicode category "Lu").

Mathematicians have found that three basic operations are needed to manipulate sets and that these operations can be chosen from a larger set of operations. In our square brackets, we already saw two of these operations: union (the square bracket is an implicit union of its atoms) and complement (a leading ^ realizes the complement of the set defined in the square bracket). W3C XML Schema extended the syntax of the Perl regular expressions to introduce a third operation: the difference between sets. The syntax follows:

[set1-[set2]]

Its meaning is all the characters in set1 that do not belong to set2, where set1 and set2 can use all the syntactic tricks that we have seen up to now.

This operator can be used to perform intersections of character classes (the intersection between two sets A and B is the difference between A and the complement of B), and we can now define a class for the BasicLatin Letters as:

[\p{IsBasicLatin}-[^\p{L}]]

Or, using the \P construct, which is also a complement, we can define the class as:

[\p{IsBasicLatin}-[\P{L}]]

The corresponding datatype definition would be:

<xs:simpleType name="BasicLatinLetters">
  <xs:restriction base="xs:token">
    <xs:pattern value="[\p{IsBasicLatin}-[\P{L}]]*"/>
  </xs:restriction>
</xs:simpleType>



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