4.24 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(ctypes) providing compatibility with some other Prolog systems. The predicates of this library are defined in terms of code_type/2.

code_type(?Code, ?Type)

char_type(?Char, ?Type)

Tests or generates alternative Types or Char/Codes. The character types are inspired by the standard C <ctype.h> primitives. The types are sensitive 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, i.e., Char is in 0 .... See also decimal.

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.

decimal

Char is a decimal digit in any script. This implies it has the Unicode general category Nd).

decimal(Weight)

Char is a decimal digit in any script with Weight 0 ....

print

Char is printable character.

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 is one of the four ISO/POSIX line-ending control codes U+000A LF, U+000B VT, U+000C FF, U+000D CR. This is the original ISO Prolog and C-string-literal definition; prolog_end_of_line is the wider set used by the SWI-Prolog reader.

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 opening bracket and Close is its matching close. Covers the three ASCII bracket pairs (), [] and {}, plus every Unicode Ps/Pe pair (about 60 pairs in Unicode 17, including angle, corner, ceiling, floor, mathematical, ornamental, fullwidth and CJK brackets). The mapping is reversible: with Close bound, Char unifies with the matching open.

quote(Close)

Char is an opening quotation mark and Close is its matching close. The ASCII quotes ’, ", and ‘ have Close = Char; Unicode Pi/Pf quote pairs (the guillemets, the standard left/right curly single and double quotes, and the single/double angle and reversed quotation marks) have Close different from Char. The mapping is reversible.

width(Width)

Width is the number of columns for fixed-width usage used by Char. True for all printable characters. Most characters require 1 column. Unicode combining characters require no space (they follow the base character). Many Asian characters and the Emojis require 2 columns. These values are used by stream_property/2 for the position(-Pos) property.

prolog_layout

Char is a Prolog layout character: a member of the Unicode Pattern_White_Space set used by read_term/2 to separate tokens. The eleven code points are U+0009..U+000D, U+0020, U+0085, U+200E, U+200F, U+2028 and U+2029. Locale- independent; pinned to unicode_syntax_version. prolog_end_of_line is the seven-element line-terminator subset.

prolog_end_of_line

Char ends a line of Prolog source text. Covers the seven line-terminator-like Pattern_White_Space code points: U+000A (LF), U+000B (VT), U+000C (FF), U+000D (CR), U+0085 (NEL), U+2028 (LINE SEPARATOR), and U+2029 (PARAGRAPH SEPARATOR). The same set terminates % comments and increments the source line counter. See section 2.15.1.9.

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.

4.24.1 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.

4.24.2 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.

4.24.3 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 5.2). 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.