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regex(7)
Standards, Environments, Macros, Character Sets, and miscellany
regex(7)
NAME
regex - internationalized basic and extended regular expression match‐
ing
DESCRIPTION
Regular Expressions (REs) provide a mechanism to select specific
strings from a set of character strings. The Internationalized Regular
Expressions described below differ from the Simple Regular Expressions
described on the regexp(7) manual page in the following ways:
o both Basic and Extended Regular Expressions are supported
o the Internationalization features—character class, equiva‐
lence class, and multi-character collation—are supported.
The Basic Regular Expression (BRE) notation and construction rules
described in the BASIC REGULAR EXPRESSIONS section apply to most
utilities supporting regular expressions. Some utilities, instead, sup‐
port the Extended Regular Expressions (ERE) described in the EXTENDED
REGULAR EXPRESSIONS section; any exceptions for both cases are noted
in the descriptions of the specific utilities using regular expres‐
sions. Both BREs and EREs are supported by the Regular Expression
Matching interfaces regcomp(3C) and regexec(3C).
BASIC REGULAR EXPRESSIONS
BREs Matching a Single Character
A BRE ordinary character, a special character preceded by a backslash,
or a period matches a single character. A bracket expression matches a
single character or a single collating element. See RE Bracket Expres‐
sion, below.
BRE Ordinary Characters
An ordinary character is a BRE that matches itself: any character in
the supported character set, except for the BRE special characters
listed in BRE Special Characters, below.
The interpretation of an ordinary character preceded by a backslash (\)
is undefined, except for:
1. the characters ), (, {, and }
2. the digits 1 to 9 inclusive (see BREs Matching Multiple
Characters, below)
3. a character inside a bracket expression.
BRE Special Characters
A BRE special character has special properties in certain contexts.
Outside those contexts, or when preceded by a backslash, such a charac‐
ter will be a BRE that matches the special character itself. The BRE
special characters and the contexts in which they have their special
meaning are:
. [ \ The period, left-bracket, and backslash are special except
when used in a bracket expression (see RE Bracket Expression,
below). An expression containing a [ that is not preceded by
a backslash and is not part of a bracket expression produces
undefined results.
* The asterisk is special except when used:
o in a bracket expression
o as the first character of an entire BRE (after an
initial ^, if any)
o as the first character of a subexpression (after
an initial ^, if any); see BREs Matching Multiple
Characters, below.
^ The circumflex is special when used:
o as an anchor (see BRE Expression Anchoring,
below).
o as the first character of a bracket expression
(see RE Bracket Expression, below).
$ The dollar sign is special when used as an anchor.
Periods in BREs
A period (.), when used outside a bracket expression, is a BRE that
matches any character in the supported character set except NUL.
RE Bracket Expression
A bracket expression (an expression enclosed in square brackets, []) is
an RE that matches a single collating element contained in the non-
empty set of collating elements represented by the bracket expression.
The following rules and definitions apply to bracket expressions:
1. A bracket expression is either a matching list expression or
a non-matching list expression. It consists of one or more
expressions: collating elements, collating symbols, equiva‐
lence classes, character classes, or range expressions (see
rule 7 below). Portable applications must not use range
expressions, even though all implementations support them.
The right-bracket (]) loses its special meaning and repre‐
sents itself in a bracket expression if it occurs first in
the list (after an initial circumflex (^), if any). Other‐
wise, it terminates the bracket expression, unless it
appears in a collating symbol (such as [.].]) or is the end‐
ing right-bracket for a collating symbol, equivalence class,
or character class. The special characters:
. * [ \
(period, asterisk, left-bracket and backslash, respectively)
lose their special meaning within a bracket expression.
The character sequences:
[. [= [:
(left-bracket followed by a period, equals-sign, or colon)
are special inside a bracket expression and are used to
delimit collating symbols, equivalence class expressions,
and character class expressions. These symbols must be fol‐
lowed by a valid expression and the matching terminating
sequence .], =] or :], as described in the following items.
2. A matching list expression specifies a list that matches any
one of the expressions represented in the list. The first
character in the list must not be the circumflex. For exam‐
ple, [abc] is an RE that matches any of the characters a, b
or c.
3. A non-matching list expression begins with a circumflex (^),
and specifies a list that matches any character or collating
element except for the expressions represented in the list
after the leading circumflex. For example, [^abc] is an RE
that matches any character or collating element except the
characters a, b, or c. The circumflex will have this spe‐
cial meaning only when it occurs first in the list, immedi‐
ately following the left-bracket.
4. A collating symbol is a collating element enclosed within
bracket-period ([..]) delimiters. Multi-character collating
elements must be represented as collating symbols when it is
necessary to distinguish them from a list of the individual
characters that make up the multi-character collating ele‐
ment. For example, if the string ch is a collating element
in the current collation sequence with the associated col‐
lating symbol <ch>, the expression [[.ch.]] will be treated
as an RE matching the character sequence ch, while [ch] will
be treated as an RE matching c or h. Collating symbols will
be recognized only inside bracket expressions. This implies
that the RE [[.ch.]]*c matches the first to fifth character
in the string chchch. If the string is not a collating ele‐
ment in the current collating sequence definition, or if the
collating element has no characters associated with it, the
symbol will be treated as an invalid expression.
5. An equivalence class expression represents the set of col‐
lating elements belonging to an equivalence class. Only pri‐
mary equivalence classes will be recognised. The class is
expressed by enclosing any one of the collating elements in
the equivalence class within bracket-equal ([==]) delim‐
iters. For example, if a and b belong to the same equiva‐
lence class, then [[=a=]b], [[==]b] and [[==]b] will each be
equivalent to [ab]. If the collating element does not belong
to an equivalence class, the equivalence class expression
will be treated as a collating symbol.
6. A character class expression represents the set of charac‐
ters belonging to a character class, as defined in the
LC_CTYPE category in the current locale. All character
classes specified in the current locale will be recognized.
A character class expression is expressed as a character
class name enclosed within bracket-colon ([::]) delimiters.
The following character class expressions are supported in
all locales:
tab(); lw(1.38i) lw(1.38i) lw(1.38i) lw(1.38i)
[:alnum:][:cntrl:][:lower:][:space:]
[:alpha:][:digit:][:print:][:upper:]
[:blank:][:graph:][:punct:][:xdigit:]
In addition, character class expressions of the form:
[:name:]
are recognized in those locales where the name keyword has
been given a charclass definition in the LC_CTYPE category.
7. A range expression represents the set of collating elements
that fall between two elements in the current collation
sequence, inclusively. It is expressed as the starting point
and the ending point separated by a hyphen (-).
Range expressions must not be used in portable applications
because their behavior is dependent on the collating
sequence. Ranges will be treated according to the current
collating sequence, and include such characters that fall
within the range based on that collating sequence, regard‐
less of character values. This, however, means that the
interpretation will differ depending on collating sequence.
If, for instance, one collating sequence defines as a vari‐
ant of a, while another defines it as a letter following z,
then the expression [-z] is valid in the first language and
invalid in the second.
In the following, all examples assume the collation sequence
specified for the POSIX locale, unless another collation
sequence is specifically defined.
The starting range point and the ending range point must be
a collating element or collating symbol. An equivalence
class expression used as a starting or ending point of a
range expression produces unspecified results. An equiva‐
lence class can be used portably within a bracket expres‐
sion, but only outside the range. For example, the unspeci‐
fied expression [[=e=]−f] should be given as [[=e=]e−f]. The
ending range point must collate equal to or higher than the
starting range point; otherwise, the expression will be
treated as invalid. The order used is the order in which the
collating elements are specified in the current collation
definition. One-to-many mappings (see locale(7)) will not be
performed. For example, assuming that the character eszet is
placed in the collation sequence after r and s, but before
t, and that it maps to the sequence ss for collation pur‐
poses, then the expression [r−s] matches only r and s, but
the expression [s−t] matches s, beta, or t.
The interpretation of range expressions where the ending
range point is also the starting range point of a subsequent
range expression (for instance [a−m−o]) is undefined.
The hyphen character will be treated as itself if it occurs
first (after an initial ^, if any) or last in the list, or
as an ending range point in a range expression. As examples,
the expressions [−ac] and [ac−] are equivalent and match any
of the characters a, c, or −; [^−ac] and [^ac−] are equiva‐
lent and match any characters except a, c, or −; the expres‐
sion [%−−] matches any of the characters between % and −
inclusive; the expression [−−@] matches any of the charac‐
ters between − and @ inclusive; and the expression [a−−@] is
invalid, because the letter a follows the symbol − in the
POSIX locale. To use a hyphen as the starting range point,
it must either come first in the bracket expression or be
specified as a collating symbol, for example: [][.−.]−0],
which matches either a right bracket or any character or
collating element that collates between hyphen and 0, inclu‐
sive.
If a bracket expression must specify both − and ], the ]
must be placed first (after the ^, if any) and the − last
within the bracket expression.
Note: Latin-1 characters such as ` or ^ are not printable in some
locales, for example, the ja locale.
BREs Matching Multiple Characters
The following rules can be used to construct BREs matching multiple
characters from BREs matching a single character:
1. The concatenation of BREs matches the concatenation of the
strings matched by each component of the BRE.
2. A subexpression can be defined within a BRE by enclosing it
between the character pairs \( and \) . Such a subexpression
matches whatever it would have matched without the \( and
\), except that anchoring within subexpressions is optional
behavior; see BRE Expression Anchoring, below. Subexpres‐
sions can be arbitrarily nested.
3. The back-reference expression \n matches the same (possibly
empty) string of characters as was matched by a subexpres‐
sion enclosed between \( and \) preceding the \n. The char‐
acter n must be a digit from 1 to 9 inclusive, nth subex‐
pression (the one that begins with the nth \( and ends with
the corresponding paired \)). The expression is invalid if
less than n subexpressions precede the \n. For example, the
expression ^\(.*\)\1$ matches a line consisting of two adja‐
cent appearances of the same string, and the expression
\(a\)*\1 fails to match a. The limit of nine back-references
to subexpressions in the RE is based on the use of a single
digit identifier. This does not imply that only nine subex‐
pressions are allowed in REs. The following is a valid BRE
with ten subexpressions:
\(\(\(ab\)*c\)*d\)\(ef\)*\(gh\)\{2\}\(ij\)*\(kl\)*\(mn\)*\(op\)*\(qr\)*
4. When a BRE matching a single character, a subexpression or a
back-reference is followed by the special character asterisk
(*), together with that asterisk it matches what zero or
more consecutive occurrences of the BRE would match. For
example, [ab]* and [ab][ab] are equivalent when matching
the string ab.
5. When a BRE matching a single character, a subexpression, or
a back-reference is followed by an interval expression of
the format \{m\}, \{m,\} or \{m,n\}, together with that
interval expression it matches what repeated consecutive
occurrences of the BRE would match. The values of m and n
will be decimal integers in the range 0 ≤ m ≤ n ≤
{RE_DUP_MAX}, where m specifies the exact or minimum number
of occurrences and n specifies the maximum number of occur‐
rences. The expression \{m\} matches exactly m occurrences
of the preceding BRE, \{m,\} matches at least m occurrences
and \{m,n\} matches any number of occurrences between m and
n, inclusive.
For example, in the string abababccccccd, the BRE c\{3\} is
matched by characters seven to nine, the BRE \(ab\)\{4,\} is
not matched at all and the BRE c\{1,3\}d is matched by char‐
acters ten to thirteen.
The behavior of multiple adjacent duplication symbols ( * and inter‐
vals) produces undefined results.
BRE Precedence
The order of precedence is as shown in the following table:
tab() box; cw(2.75i) lw(2.75i) lw(2.75i) BRE Precedence (from high to
low) collation-related bracket symbols[= =] [: :] [. .] escaped char‐
acters\<special character> bracket expression[ ] subexpressions/back-
references\( \) \n single-character-BRE duplication* \{m,n\}
concatenation anchoring^ $
BRE Expression Anchoring
A BRE can be limited to matching strings that begin or end a line; this
is called anchoring. The circumflex and dollar sign special characters
will be considered BRE anchors in the following contexts:
1. A circumflex ( ^ ) is an anchor when used as the first
character of an entire BRE. The implementation may treat
circumflex as an anchor when used as the first character of
a subexpression. The circumflex will anchor the expression
to the beginning of a string; only sequences starting at the
first character of a string will be matched by the BRE. For
example, the BRE ^ab matches ab in the string abcdef, but
fails to match in the string cdefab. A portable BRE must
escape a leading circumflex in a subexpression to match a
literal circumflex.
2. A dollar sign ( $ ) is an anchor when used as the last
character of an entire BRE. The implementation may treat a
dollar sign as an anchor when used as the last character of
a subexpression. The dollar sign will anchor the expression
to the end of the string being matched; the dollar sign can
be said to match the end-of-string following the last char‐
acter.
3. A BRE anchored by both ^ and $ matches only an entire
string. For example, the BRE ^abcdef$ matches strings con‐
sisting only of abcdef.
4. ^ and $ are not special in subexpressions.
Note: The Solaris implementation does not support anchoring in BRE sub‐
expressions.
EXTENDED REGULAR EXPRESSIONS
The rules specified for BREs apply to Extended Regular Expressions
(EREs) with the following exceptions:
o The characters |, +, and ? have special meaning, as defined
below.
o The { and } characters, when used as the duplication opera‐
tor, are not preceded by backslashes. The constructs \{ and
\} simply match the characters { and }, respectively.
o The back reference operator is not supported.
o Anchoring (^$) is supported in subexpressions.
EREs Matching a Single Character
An ERE ordinary character, a special character preceded by a backslash,
or a period matches a single character. A bracket expression matches a
single character or a single collating element. An ERE matching a
single character enclosed in parentheses matches the same as the ERE
without parentheses would have matched.
ERE Ordinary Characters
An ordinary character is an ERE that matches itself. An ordinary char‐
acter is any character in the supported character set, except for the
ERE special characters listed in ERE Special Characters below. The
interpretation of an ordinary character preceded by a backslash (\) is
undefined.
ERE Special Characters
An ERE special character has special properties in certain contexts.
Outside those contexts, or when preceded by a backslash, such a charac‐
ter is an ERE that matches the special character itself. The extended
regular expression special characters and the contexts in which they
have their special meaning are:
. [ \ ( The period, left-bracket, backslash, and left-parenthesis
are special except when used in a bracket expression (see
RE Bracket Expression, above). Outside a bracket expres‐
sion, a left-parenthesis immediately followed by a right-
parenthesis produces undefined results.
) The right-parenthesis is special when matched with a pre‐
ceding left-parenthesis, both outside a bracket expression.
* + ? { The asterisk, plus-sign, question-mark, and left-brace are
special except when used in a bracket expression (see RE
Bracket Expression, above). Any of the following uses pro‐
duce undefined results:
o if these characters appear first in an ERE, or
immediately following a vertical-line, circum‐
flex or left-parenthesis
o if a left-brace is not part of a valid interval
expression.
| The vertical-line is special except when used in a bracket
expression (see RE Bracket Expression, above). A vertical-
line appearing first or last in an ERE, or immediately fol‐
lowing a vertical-line or a left-parenthesis, or immedi‐
ately preceding a right-parenthesis, produces undefined
results.
^ The circumflex is special when used:
o as an anchor (see ERE Expression Anchoring,
below).
o as the first character of a bracket expression
(see RE Bracket Expression, above).
$ The dollar sign is special when used as an anchor.
Periods in EREs
A period (.), when used outside a bracket expression, is an ERE that
matches any character in the supported character set except NUL.
ERE Bracket Expression
The rules for ERE Bracket Expressions are the same as for Basic Regular
Expressions; see RE Bracket Expression, above).
EREs Matching Multiple Characters
The following rules will be used to construct EREs matching multiple
characters from EREs matching a single character:
1. A concatenation of EREs matches the concatenation of the
character sequences matched by each component of the ERE. A
concatenation of EREs enclosed in parentheses matches what‐
ever the concatenation without the parentheses matches. For
example, both the ERE cd and the ERE (cd) are matched by the
third and fourth character of the string abcdefabcdef.
2. When an ERE matching a single character or an ERE enclosed
in parentheses is followed by the special character plus-
sign (+), together with that plus-sign it matches what one
or more consecutive occurrences of the ERE would match. For
example, the ERE b+(bc) matches the fourth to seventh char‐
acters in the string acabbbcde; [ab] + and [ab][ab]* are
equivalent.
3. When an ERE matching a single character or an ERE enclosed
in parentheses is followed by the special character asterisk
(*), together with that asterisk it matches what zero or
more consecutive occurrences of the ERE would match. For
example, the ERE b*c matches the first character in the
string cabbbcde, and the ERE b*cd matches the third to sev‐
enth characters in the string cabbbcdebbbbbbcdbc. And, [ab]*
and [ab][ab] are equivalent when matching the string ab.
4. When an ERE matching a single character or an ERE enclosed
in parentheses is followed by the special character ques‐
tion-mark (?), together with that question-mark it matches
what zero or one consecutive occurrences of the ERE would
match. For example, the ERE b?c matches the second character
in the string acabbbcde.
5. When an ERE matching a single character or an ERE enclosed
in parentheses is followed by an interval expression of the
format {m}, {m,} or {m,n}, together with that interval
expression it matches what repeated consecutive occurrences
of the ERE would match. The values of m and n will be deci‐
mal integers in the range 0 ≤ m ≤ n ≤ {RE_DUP_MAX}, where m
specifies the exact or minimum number of occurrences and n
specifies the maximum number of occurrences. The expression
{m} matches exactly m occurrences of the preceding ERE, {m,}
matches at least m occurrences and {m,n} matches any number
of occurrences between m and n, inclusive.
For example, in the string abababccccccd the ERE c{3} is matched by
characters seven to nine and the ERE (ab){2,} is matched by characters
one to six.
The behavior of multiple adjacent duplication symbols (+, *, ? and
intervals) produces undefined results.
ERE Alternation
Two EREs separated by the special character vertical-line (|) match a
string that is matched by either. For example, the ERE a((bc)|d)
matches the string abc and the string ad. Single characters, or expres‐
sions matching single characters, separated by the vertical bar and
enclosed in parentheses, will be treated as an ERE matching a single
character.
ERE Precedence
The order of precedence will be as shown in the following table:
tab() box; cw(2.75i) lw(2.75i) lw(2.75i) ERE Precedence (from high to
low) collation-related bracket symbols[= =] [: :] [. .] escaped char‐
acters\<special character> bracket expression[ ] grouping( ) single-
character-ERE duplication* + ? {m,n} concatenation anchoring ^ $ alter‐
nation|
For example, the ERE abba|cde matches either the string abba or the
string cde (rather than the string abbade or abbcde, because concatena‐
tion has a higher order of precedence than alternation).
ERE Expression Anchoring
An ERE can be limited to matching strings that begin or end a line;
this is called anchoring. The circumflex and dollar sign special char‐
acters are considered ERE anchors when used anywhere outside a bracket
expression. This has the following effects:
1. A circumflex (^) outside a bracket expression anchors the
expression or subexpression it begins to the beginning of a
string; such an expression or subexpression can match only a
sequence starting at the first character of a string. For
example, the EREs ^ab and (^ab) match ab in the string
abcdef, but fail to match in the string cdefab, and the ERE
a^b is valid, but can never match because the a prevents the
expression ^b from matching starting at the first character.
2. A dollar sign ( $ ) outside a bracket expression anchors
the expression or subexpression it ends to the end of a
string; such an expression or subexpression can match only a
sequence ending at the last character of a string. For exam‐
ple, the EREs ef$ and (ef$) match ef in the string abcdef,
but fail to match in the string cdefab, and the ERE e$f is
valid, but can never match because the f prevents the
expression e$ from matching ending at the last character.
SEE ALSO
localedef(1), regcomp(3C), locale(7), attributes(7), environ(7), reg‐
exp(7)
Oracle Solaris 11.4 17 Mar 2016 regex(7)