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 Data Types and SubtypesData Definition (DDL) Statements 
Character Classes

A bunch of characters enclosed in brackets define a character class.A character in the string matches a class in the pattern if the character is a member of the class:

'Citroen' similar to 'Cit[arju]oen'     -- true
'Citroen' similar to 'Ci[tr]oen'        -- false
'Citroen' similar to 'Ci[tr][tr]oen'    -- true

As can be seen from the second line, the class only matches a single character, not a sequence.

Within a class definition, two characters connected by a hyphen define a range.A range comprises the two endpoints and all the characters that lie between them in the active collation.Ranges can be placed anywhere in the class definition without special delimiters to keep them apart from the other elements.

'Datte' similar to 'Dat[q-u]e'          -- true
'Datte' similar to 'Dat[abq-uy]e'       -- true
'Datte' similar to 'Dat[bcg-km-pwz]e'   -- false
Predefined Character Classes

The following predefined character classes can also be used in a class definition:

[:ALPHA:]

Latin letters a..z and A..Z.With an accent-insensitive collation, this class also matches accented forms of these characters.

[:DIGIT:]

Decimal digits 0..9.

[:ALNUM:]

Union of [:ALPHA:] and [:DIGIT:].

[:UPPER:]

Uppercase Latin letters A..Z.Also matches lowercase with case-insensitive collation and accented forms with accent-insensitive collation.

[:LOWER:]

Lowercase Latin letters a..z.Also matches uppercase with case-insensitive collation and accented forms with accent-insensitive collation.

[:SPACE:]

Matches the space character (ASCII 32).

[:WHITESPACE:]

Matches horizontal tab (ASCII 9), linefeed (ASCII 10), vertical tab (ASCII 11), formfeed (ASCII 12), carriage return (ASCII 13) and space (ASCII 32).

Including a predefined class has the same effect as including all its members.Predefined classes are only allowed within class definitions.If you need to match against a predefined class and nothing more, place an extra pair of brackets around it.

'Erdbeere' similar to 'Erd[[:ALNUM:]]eere'     -- true
'Erdbeere' similar to 'Erd[[:DIGIT:]]eere'     -- false
'Erdbeere' similar to 'Erd[a[:SPACE:]b]eere'   -- true
'Erdbeere' similar to [[:ALPHA:]]              -- false
'E'        similar to [[:ALPHA:]]              -- true

If a class definition starts with a caret, everything that follows is excluded from the class.All other characters match:

'Framboise' similar to 'Fra[^ck-p]boise'       -- false
'Framboise' similar to 'Fr[^a][^a]boise'       -- false
'Framboise' similar to 'Fra[^[:DIGIT:]]boise'  -- true

If the caret is not placed at the start of the sequence, the class contains everything before the caret, except for the elements that also occur after the caret:

'Grapefruit' similar to 'Grap[a-m^f-i]fruit'   -- true
'Grapefruit' similar to 'Grap[abc^xyz]fruit'   -- false
'Grapefruit' similar to 'Grap[abc^de]fruit'    -- false
'Grapefruit' similar to 'Grap[abe^de]fruit'    -- false

'3' similar to '[[:DIGIT:]^4-8]'               -- true
'6' similar to '[[:DIGIT:]^4-8]'               -- false

Lastly, the already mentioned wildcard ‘_’ is a character class of its own, matching any single character.