stringi-search-charclass: Character Classes in stringi

Description

In this man page we describe how character classes are declared in the stringi package so that you may e.g. find their occurrences in your search activities or generate random codepoints 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 either bounded by square brackets (such patterns follow a syntax similar to that employed by version 8 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''. On the other hand, [[pat1][pat2]], [[pat1]&[pat2]], and [[pat1]-[pat2]] denote union, intersection, and asymmetric diference of sets specified by pat1 and pat2, respectively.

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

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

A malformed pattern always results in an error.

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 properties' 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.Lufor 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 perfectly documented. Please refer to the References section for some links.

Please note that some classes may seem to 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 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 the complete 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, cf. alsoASCII_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.

and many more.

Details

All stri_*_charclass functions in stringi perform a single character (Unicode codepoint) search-based operations. Since stringi_0.2-1 you may obtain roughly the same results using stringi-search-regex. However, these very functions aim to be quite 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, http://www.unicode.org/reports/tr44/

UnicodeSet -- ICU User Guide, http://userguide.icu-project.org/strings/unicodeset

Properties -- ICU User Guide, http://userguide.icu-project.org/strings/properties

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

icu::Unicodeset Class Reference -- ICU4C API Documentation, http://www.icu-project.org/apiref/icu4c/classicu_1_1UnicodeSet.html