REGEX(5) Standards, Environments, and Macros REGEX(5)

NAME


regex - internationalized basic and extended regular expression matching

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:

+o both Basic and Extended Regular Expressions are supported

+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 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 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, equivalence 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 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 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
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 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 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 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]", "[[==]a]" 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 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:

[: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, 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
example, 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 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
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 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 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
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
whatever it would have matched without the "\(" and Qq \) , 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
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
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.

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 <= BRE_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 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
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 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 operator, 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 character
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 character
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 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, 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 produce 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
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 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 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 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 character 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 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 Ns}
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 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
"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$)"
match "ef" in the string "abcdef", but fail to match in the string
"cdefab", and the ERE "e$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)

illumos June 2, 2017 illumos