4.22 Character properties

SWI-Prolog offers two comprehensive predicates for classifying characters and character codes. These predicates are defined as built-in predicates to exploit the C-character classification's handling of locale (handling of local character sets). These predicates are fast, logical and deterministic if applicable.

In addition, there is the library library(ctype) providing compatibility with some other Prolog systems. The predicates of this library are defined in terms of code_type/2.

char_type(?Char, ?Type)

Tests or generates alternative Types or Chars. The character types are inspired by the standard C <ctype.h> primitives.

alnum

Char is a letter (upper- or lowercase) or digit.

alpha

Char is a letter (upper- or lowercase).

csym

Char is a letter (upper- or lowercase), digit or the underscore (_). These are valid C and Prolog symbol characters.

csymf

Char is a letter (upper- or lowercase) or the underscore (_). These are valid first characters for C and Prolog symbols.

ascii

Char is a 7-bit ASCII character (0..127).

white

Char is a space or tab, i.e. white space inside a line.

cntrl

Char is an ASCII control character (0..31).

digit

Char is a digit.

digit(Weight)

Char is a digit with value Weight. I.e. char_type(X, digit(6) yields X = '6'. Useful for parsing numbers.

xdigit(Weight)

Char is a hexadecimal digit with value Weight. I.e. char_type(a, xdigit(X) yields X = '10'. Useful for parsing numbers.

graph

Char produces a visible mark on a page when printed. Note that the space is not included!

lower

Char is a lowercase letter.

lower(Upper)

Char is a lowercase version of Upper. Only true if Char is lowercase and Upper uppercase.

to_lower(Upper)

Char is a lowercase version of Upper. For non-letters, or letter without case, Char and Lower are the same. See also upcase_atom/2 and downcase_atom/2.

upper

Char is an uppercase letter.

upper(Lower)

Char is an uppercase version of Lower. Only true if Char is uppercase and Lower lowercase.

to_upper(Lower)

Char is an uppercase version of Lower. For non-letters, or letter without case, Char and Lower are the same. See also upcase_atom/2 and downcase_atom/2.

punct

Char is a punctuation character. This is a graph character that is not a letter or digit.

space

Char is some form of layout character (tab, vertical tab, newline, etc.).

end_of_file

Char is -1.

end_of_line

Char ends a line (ASCII: 10..13).

newline

Char is a newline character (10).

period

Char counts as the end of a sentence (.,!,?).

quote

Char is a quote character (", ', `).

paren(Close)

Char is an open parenthesis and Close is the corresponding close parenthesis.

code_type(?Code, ?Type)

As char_type/2, but uses character codes rather than one-character atoms. Please note that both predicates are as flexible as possible. They handle either representation if the argument is instantiated and will instantiate only with an integer code or a one-character atom, depending of the version used. See also the Prolog flag double_quotes, atom_chars/2 and atom_codes/2.

4.22.1 Case conversion

There is nothing in the Prolog standard for converting case in textual data. The SWI-Prolog predicates code_type/2 and char_type/2 can be used to test and convert individual characters. We have started some additional support:

downcase_atom(+AnyCase, -LowerCase)

Converts the characters of AnyCase into lowercase as char_type/2 does (i.e. based on the defined locale if Prolog provides locale support on the hosting platform) and unifies the lowercase atom with LowerCase.

upcase_atom(+AnyCase, -UpperCase)

Converts, similar to downcase_atom/2, an atom to uppercase.

4.22.2 White space normalization

normalize_space(-Out, +In)

Normalize white space in In. All leading and trailing white space is removed. All non-empty sequences for Unicode white space characters are replaced by a single space (\u0020) character. Out uses the same conventions as with_output_to/2 and format/3.

4.22.3 Language-specific comparison

This section deals with predicates for language-specific string comparison operations.

collation_key(+Atom, -Key)

Create a Key from Atom for locale-specific comparison. The key is defined such that if the key of atom A precedes the key of atom B in the standard order of terms, A is alphabetically smaller than B using the sort order of the current locale.

The predicate collation_key/2 is used by locale_sort/2 from library(sort). Please examine the implementation of locale_sort/2 as an example of using this call.

The Key is an implementation-defined and generally unreadable string. On systems that do not support locale handling, Key is simply unified with Atom.

locale_sort(+List, -Sorted)

Sort a list of atoms using the current locale. List is a list of atoms or string objects (see section 4.23). Sorted is unified with a list containing all atoms of List, sorted to the rules of the current locale. See also collation_key/2 and setlocale/3.