about_search_charclass: Character Classes in stringi

Description

Here we describe how character classes (sets) can be specified in the stringi package. These are useful for defining search patterns (note that the ICU regex engine uses the same scheme for denoting character classes) or, e.g., generating random code points with stri_rand_strings.

Arguments

<code>UnicodeSet</code> patterns

A UnicodeSet represents a subset of Unicode code points (recall that stringi converts strings in your native encoding to Unicode automatically). Legal code points are U+0000 to U+10FFFF, inclusive.

Patterns either consist of series of characters bounded by square brackets (such patterns follow a syntax similar to that employed by regular expression character classes) or of Perl-like Unicode property set specifiers.

[] denotes an empty set, [a] -- a set consisting of character ``a'', [\u0105] -- a set with character U+0105, and [abc] -- a set with ``a'', ``b'', and ``c''.

[a-z] denotes a set consisting of characters ``a'' through ``z'' inclusively, in Unicode code point order.

Some set-theoretic operations are available. ^ denotes the complement, e.g., [^a-z] contains all characters but ``a'' through ``z''. Moreover, [[pat1][pat2]], [[pat1]\&[pat2]], and [[pat1]-[pat2]] denote union, intersection, and asymmetric difference of sets specified by pat1 and pat2, respectively.

Note that all white-spaces are ignored unless they are quoted or back-slashed (white spaces can be freely used for clarity, as [a c d-f m] means the same as [acd-fm]). stringi does not allow including multi-character strings (see UnicodeSet API documentation). Also, empty string patterns are disallowed.

Any character may be preceded by a backslash in order to remove its special meaning.

A malformed pattern always results in an error.

Set expressions at a glance (according to https://unicode-org.github.io/icu/userguide/strings/regexp.html):

Some examples:

[abc]: Match any of the characters a, b or c.
[^abc]: Negation -- match any character except a, b or c.
[A-M]: Range -- match any character from A to M. The characters to include are determined by Unicode code point ordering.
[\u0000-\U0010ffff]: Range -- match all characters.
[\p{Letter}] or [\p{General_Category=Letter}] or [\p{L}]: Characters with Unicode Category = Letter. All forms shown are equivalent.
[\P{Letter}]: Negated property (Note the upper case \P) -- match everything except Letters.
[\p{numeric_value=9}]: Match all numbers with a numeric value of 9. Any Unicode Property may be used in set expressions.
[\p{Letter}&\p{script=cyrillic}]: Set intersection -- match the set of all Cyrillic letters.
[\p{Letter}-\p{script=latin}]: Set difference -- match all non-Latin letters.
[[a-z][A-Z][0-9]] or [a-zA-Z0-9]: Implicit union of sets -- match ASCII letters and digits (the two forms are equivalent).
[:script=Greek:]: Alternative POSIX-like syntax for properties -- equivalent to \p{script=Greek}.

Unicode properties

Unicode property sets are specified with a POSIX-like syntax, e.g., [:Letter:], or with a (extended) Perl-style syntax, e.g., \p{L}. The complements of the above sets are [:^Letter:] and \P{L}, respectively.

The names are normalized before matching (for example, the match is case-insensitive). Moreover, many names have short aliases.

Among predefined Unicode properties we find, e.g.:

Unicode General Categories, e.g., Lu for uppercase letters,
Unicode Binary Properties, e.g., WHITE_SPACE,

and many more (including Unicode scripts).

Each property provides access to the large and comprehensive Unicode Character Database. Generally, the list of properties available in ICU is not well-documented. Please refer to the References section for some links.

Please note that some classes might overlap. However, e.g., General Category Z (some space) and Binary Property WHITE_SPACE matches different character sets.

Unicode General Categories

The Unicode General Category property of a code point provides the most general classification of that code point. Each code point falls into one and only one Category.

Cc: a C0 or C1 control code.
Cf: a format control character.
Cn: a reserved unassigned code point or a non-character.
Co: a private-use character.
Cs: a surrogate code point.
Lc: the union of Lu, Ll, Lt.
Ll: a lowercase letter.
Lm: a modifier letter.
Lo: other letters, including syllables and ideographs.
Lt: a digraphic character, with the first part uppercase.
Lu: an uppercase letter.
Mc: a spacing combining mark (positive advance width).
Me: an enclosing combining mark.
Mn: a non-spacing combining mark (zero advance width).
Nd: a decimal digit.
Nl: a letter-like numeric character.
No: a numeric character of other type.
Pd: a dash or hyphen punctuation mark.
Ps: an opening punctuation mark (of a pair).
Pe: a closing punctuation mark (of a pair).
Pc: a connecting punctuation mark, like a tie.
Po: a punctuation mark of other type.
Pi: an initial quotation mark.
Pf: a final quotation mark.
Sm: a symbol of mathematical use.
Sc: a currency sign.
Sk: a non-letter-like modifier symbol.
So: a symbol of other type.
Zs: a space character (of non-zero width).
Zl: U+2028 LINE SEPARATOR only.
Zp: U+2029 PARAGRAPH SEPARATOR only.
C: the union of Cc, Cf, Cs, Co, Cn.
L: the union of Lu, Ll, Lt, Lm, Lo.
M: the union of Mn, Mc, Me.
N: the union of Nd, Nl, No.
P: the union of Pc, Pd, Ps, Pe, Pi, Pf, Po.
S: the union of Sm, Sc, Sk, So.
Z: the union of Zs, Zl, Zp

Unicode Binary Properties

Each character may follow many Binary Properties at a time.

Here is a comprehensive list of supported Binary Properties:

ALPHABETIC: alphabetic character.
ASCII_HEX_DIGIT: a character matching the [0-9A-Fa-f] charclass.
BIDI_CONTROL: a format control which have specific functions in the Bidi (bidirectional text) Algorithm.
BIDI_MIRRORED: a character that may change display in right-to-left text.
DASH: a kind of a dash character.
DEFAULT_IGNORABLE_CODE_POINT: characters that are ignorable in most text processing activities, e.g., <2060..206F, FFF0..FFFB, E0000..E0FFF>.
DEPRECATED: a deprecated character according to the current Unicode standard (the usage of deprecated characters is strongly discouraged).
DIACRITIC: a character that linguistically modifies the meaning of another character to which it applies.
EXTENDER: a character that extends the value or shape of a preceding alphabetic character, e.g., a length and iteration mark.
HEX_DIGIT: a character commonly used for hexadecimal numbers, see also ASCII_HEX_DIGIT.
HYPHEN: a dash used to mark connections between pieces of words, plus the Katakana middle dot.
ID_CONTINUE: a character that can continue an identifier, ID_START+Mn+Mc+Nd+Pc.
ID_START: a character that can start an identifier, Lu+Ll+Lt+Lm+Lo+Nl.
IDEOGRAPHIC: a CJKV (Chinese-Japanese-Korean-Vietnamese) ideograph.
LOWERCASE: ...
MATH: ...
NONCHARACTER_CODE_POINT: ...
QUOTATION_MARK: ...
SOFT_DOTTED: a character with a ``soft dot'', like i or j, such that an accent placed on this character causes the dot to disappear.
TERMINAL_PUNCTUATION: a punctuation character that generally marks the end of textual units.
UPPERCASE: ...
WHITE_SPACE: a space character or TAB or CR or LF or ZWSP or ZWNBSP.
CASE_SENSITIVE: ...
POSIX_ALNUM: ...
POSIX_BLANK: ...
POSIX_GRAPH: ...
POSIX_PRINT: ...
POSIX_XDIGIT: ...
CASED: ...
CASE_IGNORABLE: ...
CHANGES_WHEN_LOWERCASED: ...
CHANGES_WHEN_UPPERCASED: ...
CHANGES_WHEN_TITLECASED: ...
CHANGES_WHEN_CASEFOLDED: ...
CHANGES_WHEN_CASEMAPPED: ...
CHANGES_WHEN_NFKC_CASEFOLDED: ...
EMOJI: Since ICU 57
EMOJI_PRESENTATION: Since ICU 57
EMOJI_MODIFIER: Since ICU 57
EMOJI_MODIFIER_BASE: Since ICU 57

POSIX Character Classes

Avoid using POSIX character classes, e.g., [:punct:]. The ICU User Guide (see below) states that in general they are not well-defined, so you may end up with something different than you expect.

In particular, in POSIX-like regex engines, [:punct:] stands for the character class corresponding to the ispunct() classification function (check out man 3 ispunct on UNIX-like systems). According to ISO/IEC 9899:1990 (ISO C90), the ispunct() function tests for any printing character except for space or a character for which isalnum() is true. However, in a POSIX setting, the details of what characters belong into which class depend on the current locale. So the [:punct:] class does not lead to a portable code (again, in POSIX-like regex engines).

Therefore, a POSIX flavor of [:punct:] is more like [\p{P}\p{S}] in ICU. You have been warned.

Author

Marek Gagolewski and other contributors

Details

All stri_*_charclass functions in stringi perform a single character (i.e., Unicode code point) search-based operations. You may obtain the same results using about_search_regex. However, these very functions aim to be faster.

Character classes are defined using ICU's UnicodeSet patterns. Below we briefly summarize their syntax. For more details refer to the bibliographic References below.

References

The Unicode Character Database -- Unicode Standard Annex #44, https://www.unicode.org/reports/tr44/

UnicodeSet -- ICU User Guide, https://unicode-org.github.io/icu/userguide/strings/unicodeset.html

Properties -- ICU User Guide, https://unicode-org.github.io/icu/userguide/strings/properties.html

C/POSIX Migration -- ICU User Guide, https://unicode-org.github.io/icu/userguide/icu/posix.html

Unicode Script Data, https://www.unicode.org/Public/UNIDATA/Scripts.txt

icu::Unicodeset Class Reference -- ICU4C API Documentation, https://unicode-org.github.io/icu-docs/apidoc/dev/icu4c/classicu_1_1UnicodeSet.html