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Standards, Environments, and Macros regex(5)
NAME
regex - internationalized basic and extended regular expres-
sion matching
DESCRIPTION
Regular Expressions (REs) provide a mechanism to select
specific strings from a set of character strings. The Inter-
nationalized Regular Expressions described below differ from
the Simple Regular Expressions described on the regexp(5)
manual page in the following ways:
o both Basic and Extended Regular Expressions are sup-
ported
o the Internationalization features-character class,
equivalence 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, support 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 expressions. Both
BREs and EREs are supported by the Regular Expression Match-
ing 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 Expression, 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.
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Standards, Environments, and Macros regex(5)
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 character 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 contain-
ing 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 Match-
ing 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.
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Standards, Environments, and Macros regex(5)
RE Bracket Expression
A bracket expression (an expression enclosed in square
brackets, []) is an RE that matches a single collating ele-
ment contained in the non-empty set of collating elements
represented by the bracket expression.
The following rules and definitions apply to bracket expres-
sions:
1. A bracket expression is either a matching list expres-
sion or a non-matching list expression. It consists of
one or more expressions: collating elements, collating
symbols, equivalence classes, character classes, or
range expressions (see rule 7 below). Portable applica-
tions must not use range expressions, even though all
implementations support them. The right-bracket (])
loses its special meaning and represents itself in a
bracket expression if it occurs first in the list (after
an initial circumflex (^), if any). Otherwise, it ter-
minates the bracket expression, unless it appears in a
collating symbol (such as [.].]) or is the ending
right-bracket for a collating symbol, equivalence class,
or character class. The special characters:
. * [ \
(period, asterisk, left-bracket and backslash, respec-
tively) 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 sym-
bols must be followed 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 example, [abc] is an RE that matches any of the
characters
a, b or c.
3.
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Standards, Environments, and Macros regex(5)
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 special meaning only when it occurs first
in the list, immediately 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 sym-
bols when it is necessary to distinguish them from a
list of the individual characters that make up the
multi-character collating element. For example, if the
string ch is a collating element in the current colla-
tion sequence with the associated collating symbol <ch>,
the expression [[.ch.]] will be treated as an RE match-
ing 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 element 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
collating elements belonging to an equivalence class.
Only primary equivalence classes will be recognised. The
class is expressed by enclosing any one of the collating
elements in the equivalence class within bracket-equal
([==]) delimiters. For example, if a and b belong to the
same equivalence class, then [[=a=]b], [[==]b] and
[[==]b] will each be equivalent to [ab]. If the collat-
ing element does not belong to an equivalence class, the
equivalence class expression will be treated as a col-
lating symbol.
6. A character class expression represents the set of char-
acters 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 recog-
nized. A character class expression is expressed as a
character class name enclosed within bracket-colon
([::]) delimiters.
The following character class expressions are supported
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Standards, Environments, and Macros regex(5)
in all locales:
[: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 ele-
ments that fall between two elements in the current col-
lation 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 applica-
tions because their behavior is dependent on the collat-
ing 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, regardless of character values. This, however,
means that the interpretation will differ depending on
collating sequence. If, for instance, one collating
sequence defines as a variant 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 end-
ing point of a range expression produces unspecified
results. An equivalence class can be used portably
within a bracket expression, but only outside the range.
For example, the unspecified expression [[=e=]-f] should
be given as [[=e=]e-f]. The ending range point must col-
late 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 ele-
ments are specified in the current collation definition.
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Standards, Environments, and Macros regex(5)
One-to-many mappings (see locale(5)) will not be per-
formed. 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 colla-
tion purposes, 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 subse-
quent range expression (for instance [a-m-o]) is unde-
fined.
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 equivalent and match any charac-
ters except a, c, or -; the expression [%--] matches any
of the characters between % and - inclusive; the expres-
sion [--@] matches any of the characters 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 speci-
fied 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,
inclusive.
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 subex-
pression matches whatever it would have matched without
the \( and \), except that anchoring within subexpres-
sions is optional behavior; see BRE Expression Anchor-
ing, below. Subexpressions can be arbitrarily nested.
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Standards, Environments, and Macros regex(5)
3. The back-reference expression \n matches the same (pos-
sibly empty) string of characters as was matched by a
subexpression enclosed between \( and \) preceding the
\n. The character n must be a digit from 1 to 9
inclusive, nth subexpression (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 adjacent appearances of
the same string, and the expression \(a\)*\1 fails to
match a. The limit of nine back-references to subexpres-
sions 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 expres-
sion 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 max-
imum number of occurrences. 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 characters ten to thirteen.
The behavior of multiple adjacent duplication symbols ( *
and intervals) produces undefined results.
BRE Precedence
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Standards, Environments, and Macros regex(5)
The order of precedence is as shown in the following table:
_________________________________________________________________
| BRE Precedence (from high to low) |
| collation-related bracket symbols| [= =] [: :] [. .] |
| escaped characters | \<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 charac-
ter 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 fol-
lowing the last character.
3. A BRE anchored by both ^ and $ matches only an entire
string. For example, the BRE ^abcdef$ matches strings
consisting only of abcdef.
4. ^ and $ are not special in subexpressions.
Note: The Solaris implementation does not support anchoring
in BRE subexpressions.
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Standards, Environments, and Macros regex(5)
EXTENDED REGULAR EXPRESSIONS
The rules specififed 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
operator, are not preceded by backslashes. The con-
structs \{ 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 character is any character in the supported charac-
ter 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 character is an ERE that matches the spe-
cial character itself. The extended regular expression spe-
cial 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
expression, a left-parenthesis immediately
followed by a right-parenthesis produces
undefined results.
) The right-parenthesis is special when
matched with a preceding left-parenthesis,
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Standards, Environments, and Macros regex(5)
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 Expres-
sion, 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, circumflex 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 appear-
ing first or last in an ERE, or immediately
following a vertical-line or a left-
parenthesis, or immediately 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
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Standards, Environments, and Macros regex(5)
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 whatever the concatenation without the
parentheses matches. For example, both the ERE cd and
the ERE (cd) are matched by the third and fourth charac-
ter of the string abcdefabcdef.
2. When an ERE matching a single character or an ERE
enclosed in parentheses is followed by the special char-
acter 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 characters 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 char-
acter 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 seventh 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 char-
acter question-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
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Standards, Environments, and Macros regex(5)
consecutive occurrences of the ERE 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 max-
imum 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 expressions matching single charac-
ters, 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:
_________________________________________________________________
| ERE Precedence (from high to low) |
| collation-related bracket symbols| [= =] [: :] [. .] |
| escaped characters | \<special character> |
| bracket expression | [ ] |
| grouping | ( ) |
| single-character-ERE duplication | * + ? {m,n} |
| concatenation | |
| anchoring | ^ $ |
| alternation | | |
|__________________________________|_____________________________|
For example, the ERE abba|cde matches either the string abba
or the string cde (rather than the string abbade or abbcde,
because concatenation 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
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Standards, Environments, and Macros regex(5)
sign special characters are considered ERE anchors when used
anywhere outside a bracket expression. This has the follow-
ing effects:
1. A circumflex (^) outside a bracket expression anchors
the expression or subexpression it begins to the begin-
ning of a string; such an expression or subexpression
can match only a sequence starting at the first charac-
ter 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 example, the EREs ef$ and (ef$) match ef in
the string abcdef, but fail to match in the string cde-
fab, and the ERE e$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), attributes(5), environ(5),
locale(5), regexp(5)
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