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 sec-
tion; 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 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. 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 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 Multi-
ple 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 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 expression
or a non-matching list expression. It consists of one or
more expressions: collating elements, collating symbols,
equivalence classes, character classes, or range expres-
sions (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 represents itself in a bracket expression if
it occurs first in the list (after an initial circumflex
(^), if any). Otherwise, 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, 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 char-
acters
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 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 collat-
ing 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 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 recog-
nized 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 expres-
sion.
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 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 collat-
ing 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
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 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 collating elements are speci-
fied 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 collation 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 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 characters
except a, c, or -; the expression [%--] matches any of
the characters 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 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 Anchoring,
below. Subexpressions can be arbitrarily nested.
3. The back-reference expression \n matches the same (possi-
bly 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 subexpres-
sions 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 con-
secutive 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 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
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.
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). Out-
side a bracket expression, a left-parenthesis immedi-
ately followed by a right-parenthesis produces unde-
fined 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 fol-
lowing uses produce 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 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
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 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
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 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 pre-
cedence 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 characters 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 cde-
fab, 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$) match ef in the
string abcdef, but fail to match in the string cdefab,
and the ERE e$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), attributes(5), environ(5),
locale(5), regexp(5)
Man(1) output converted with
man2html