6.4.3.1. Classical Perl character classes

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

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

6.4.3.2. Unicode character classes

Table 6-2. Unicode character classes

<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

[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.

[-^\\]

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").

[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.

[\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>