<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 difference 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. Set expressions at a glance
(according to http://userguide.icu-project.org/strings/regexp): 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.
(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}]- Logical AND
or intersection -- match the set of all Cyrillic letters.
[\p{Letter}--\p{script=latin}]- Subtraction --
match all non-Latin letters.
[[a-z][A-Z][0-9]] or [a-zA-Z0-9]- Implicit Logical
OR or Union of Sets -- the examples match ASCII letters and digits.
The two forms are equivalent.
[:script=Greek:]- Alternate 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 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.
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 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 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,
cf. 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.
LOWERCASEMATHNONCHARACTER_CODE_POINTQUOTATION_MARKSOFT_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.
UPPERCASEWHITE_SPACE- a space character or TAB or CR or LF or ZWSP or ZWNBSP.
CASE_SENSITIVEPOSIX_ALNUMPOSIX_BLANKPOSIX_GRAPHPOSIX_PRINTPOSIX_XDIGITCASEDCASE_IGNORABLECHANGES_WHEN_LOWERCASEDCHANGES_WHEN_UPPERCASEDCHANGES_WHEN_TITLECASEDCHANGES_WHEN_CASEFOLDEDCHANGES_WHEN_CASEMAPPEDCHANGES_WHEN_NFKC_CASEFOLDED
POSIX Character Classes
Beware of using POSIX character classes,
e.g. [:punct:]. ICU User Guide (see below)
states that in general they are not well-defined, so 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 portable code (again, in POSIX-like regex engines). So a POSIX flavor of [:punct:] is more like
[\p{P}\p{S}] in ICU. You have been warned.