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 ... 9. 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 ... 9.

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.

prolog_solo

Char is a Prolog solo character: a punctuation code point that forms an atom on its own and never combines with neighbouring symbol characters. In ASCII the solo set is !, ; and %; the same flag carries over to non-ASCII code points via the Unicode syntax map (see section 2.15.1.9). Solo characters are written unquoted by writeq/1 and are accepted as single-character atoms by the reader.

pattern_syntax

Char has the Unicode Pattern_Syntax property (UAX #31 R3). This is the immutable set of punctuation and symbol code points whose classification is guaranteed not to change across Unicode versions. Used by write_canonical/1 and the pattern_syntax_solo option of write_term/2 to decide which single-character atoms can be printed bare with round-trip safety across Unicode upgrades.