regex(5) Standards, Environments, and Macros regex(5)NAMEregex - 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(5) manual page in the following ways:
· both Basic and Extended Regular Expressions are supported
· 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 utili‐
ties supporting regular expressions. Some utilities, instead, support
the Extended Regular Expressions (ERE) described in the EXTENDED REGU‐
LAR 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 Matching inter‐
faces 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:
· in a bracket expression
· as the first character of an entire BRE (after an initial
^, if any)
· as the first character of a subexpression (after an ini‐
tial ^, if any); see BREs Matching Multiple Characters,
below.
^ The circumflex is special when used:
· as an anchor (see BRE Expression Anchoring, below).
· 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, equivalence classes, charac‐
ter classes, or range expressions (see rule 7 below). Portable
applications must not use range expressions, even though all imple‐
mentations 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). Other‐
wise, it terminates 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 spe‐
cial 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 spe‐
cial inside a bracket expression and are used to delimit collating
symbols, equivalence class expressions, and character class expres‐
sions. These symbols 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. 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 lead‐
ing circumflex. For example, [^abc] is an RE that matches any char‐
acter 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 symbols when it is necessary to distin‐
guish 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 collation sequence with
the associated collating 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 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 equiva‐
lence classes will be recognised. The class is expressed by enclos‐
ing 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 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 characters
belonging to a character class, as defined in the LC_CTYPE category
in the current locale. All character classes specified in the cur‐
rent 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:
[: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, inclu‐
sively. 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, 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 expres‐
sion [-z] is valid in the first language and invalid in the second.
In the following, all examples assume the collation sequence speci‐
fied for the POSIX locale, unless another collation sequence is
specifically defined.
The starting range point and the ending range point must be a col‐
lating element or collating symbol. An equivalence class expression
used as a starting or ending 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 exam‐
ple, the unspecified 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 col‐
lating elements are specified in the current collation definition.
One-to-many mappings (see locale(5)) will not be performed. For
example, assuming that the character eszet is placed in the colla‐
tion sequence after r and s, but before t, and that it maps to the
sequence ss for collation 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 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 equivalent and match any characters
except a, c, or −; the expression [%−−] matches any of the charac‐
ters between % and − inclusive; the expression [−−@] 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 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, 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 subexpression matches what‐
ever it would have matched without the \( and \), except that
anchoring within subexpressions is optional behavior; see BRE
Expression Anchoring, below. Subexpressions can be arbitrarily
nested.
3. The back-reference expression \n matches the same (possibly 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 expres‐
sion 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
subexpressions in the RE is based on the use of a single digit
identifier. This does not imply that only nine subexpressions are
allowed in REs. The following is a valid BRE with ten subexpres‐
sions:
\(\(\(ab\)*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 consecu‐
tive 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 characters 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:
┌────────────────────────────────────────────────────────────────┐
│ 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 character of a subexpression. The dol‐
lar 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 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 sub‐
expressions.
EXTENDED REGULAR EXPRESSIONS
The rules specififed for BREs apply to Extended Regular Expressions
(EREs) with the following exceptions:
· The characters |, +, and ? have special meaning, as defined below.
· The { and } characters, when used as the duplication operator, are
not preceded by backslashes. The constructs \{ and \} simply match
the characters { and }, respectively.
· The back reference operator is not supported.
· 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 sin‐
gle character enclosed in parentheses matches the same as the ERE with‐
out 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-parenthe‐
sis are special except when used in a bracket expres‐
sion (see RE Bracket Expression, above). Outside a
bracket expression, a left-parenthesis immediately fol‐
lowed by a right-parenthesis produces undefined
results.
) The right-parenthesis is special when matched with a
preceding 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 follow‐
ing uses produce undefined results:
· if these characters appear first in an ERE, or
immediately following a vertical-line, circumflex
or left-parenthesis
· 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 following a vertical-line or a left-paren‐
thesis, or immediately preceding a right-parenthesis,
produces undefined results.
^ The circumflex is special when used:
· as an anchor (see ERE Expression Anchoring,
below).
· 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 whatever 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 consecu‐
tive 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 character asterisk (*),
together with that asterisk it matches what zero or more consecu‐
tive 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 cabb‐
bcdebbbbbbcdbc. And, [ab]* and [ab][ab] are equivalent when match‐
ing the string ab.
4. When an ERE matching a single character or an ERE enclosed in
parentheses is followed by the special character question-mark (?),
together with that question-mark it matches what zero or one con‐
secutive 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 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 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:
┌────────────────────────────────────────────────────────────────┐
│ 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 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 expres‐
sion 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 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 charac‐
ter.
SEE ALSOlocaledef(1), regcomp(3C), attributes(5), environ(5), locale(5), reg‐
exp(5)SunOS 5.10 21 Apr 2005 regex(5)
