SWI-Prolog -- char

Documentation
- Reference manual
  - Built-in Predicates
    - Character properties
- Packages

Availability:built-in

char_type(?Char, ?Type)

Tests or generates alternative Types or Chars. The character types are inspired by the standard C <ctype.h> primitives. The types are sensititve to the active locale, see setlocale/3. Most of the Types are mapped to the Unicode classification functions from <wctype.h>, e.g., alnum uses iswalnum(). The types prolog_var_start, prolog_atom_start, prolog_identifier_continue and prolog_symbol are based on the locale-independent built-in classification routines that are also used by read/1 and friends.

Note that the mode (-,+) is only efficient if the Type has a parameter, e.g., char_type(C, digit(8)). If Type is a atomic, the whole unicode range (0..0x1ffff) is generated and tested against the character classification function.

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), ASCII DEL character (127), or non-ASCII character in the range 128..159 or 8232..8233.
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.
prolog_var_start: Char can start a Prolog variable name.
prolog_atom_start: Char can start a unquoted Prolog atom that is not a symbol.
prolog_identifier_continue: Char can continue a Prolog variable name or atom.
prolog_symbol: Char is a Prolog symbol character. Sequences of Prolog symbol characters glue together to form an unquoted atom. Examples are =.., \=, etc.