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'\"! tbl | mmdoc
'\"macro stdmacro
.TH REGCOMP 5
.SH NAME
regcomp \- X/Open regular expressions definition and interface
.SH DESCRIPTION
\f4Note:\fP Two versions of regular expressions are supported:
.in+.5i
.TP .5i
\f4o\fP
the historical \f4Simple Regular Expressions\fP,
which provide backward compatibility, but which will be withdrawn from a
future issue of this document set
.TP .5i
\f4o\fP
the improved internationalised version that complies with the
ISO/IEC 9945-2: 1993 standard.
.in-.5i
.sp
The first (historical) version is described as part of the
\f2regexp\fP function in the \f4regexp(5)\fP man page. The second (improved)
version is described in this man page.
.PP
.I "Regular Expressions"
(REs) provide a mechanism to select specific strings from a set of character
strings.
.PP
Regular expressions are a context-independent syntax that can represent a
wide variety of character sets and character set orderings, where these
character sets are interpreted according to the current locale. While many
regular expressions can be interpreted differently depending on the current
locale, many features, such as character class expressions, provide for
contextual invariance across locales.
.PP
The Basic Regular Expression (BRE) notation and construction rules in
\f4bre\fP apply to most utilities supporting regular expressions. Some
utilities, instead, support the Extended Regular Expressions (ERE) described
in \f4ere\fP; 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 interface in the \f4regcmp()\fP,
\f4regexec()\fP and related functions.
.SH "Regular Expression Definitions"
.PP
For the purposes of this section, the following definitions apply:
.sp
\f4entire regular expression\fP
.PP
The concatenated set of one or more BREs or EREs that make up the pattern
specified for string selection.
.sp
\f4matched\fP
.PP
.in+0.5i
A sequence of zero or more characters is said to be matched by a BRE or ERE
when the characters in the sequence correspond to a sequence of characters
defined by the pattern.
.sp
Matching is based on the bit pattern used for encoding the character, not on
the graphic representation of the character. This means that if a character
set contains two or more encodings for a graphic symbol, or if the strings
searched contain text encoded in more than one codeset, no attempt is made
to search for any other representation of the encoded symbol. If that is
required, the user can specify equivalence classes containing all variations
of the desired graphic symbol.
.sp
The search for a matching sequence starts at the beginning of a string and
stops when the first sequence matching the expression is found, where
\f2first\fP is defined to mean ``begins earliest in the string''. If the
pattern permits a variable number of matching characters and thus there is
more than one such sequence starting at that point, the longest such
sequence will be matched. For example: the BRE bb* matches the second to
fourth characters of abbbc, and the ERE (wee|week)(knights|night) matches
all ten characters of weeknights.
.sp
Consistent with the whole match being the longest of the leftmost matches,
each subpattern, from left to right, matches the longest possible string.
For this purpose, a null string is considered to be longer than no match
at all. For example, matching the BRE \\(.*\\).* against abcdef, the
subexpression (\\1) is abcdef, and matching the BRE \\(a*\\)* against bc, the
subexpression (\\1) is the null string.
.sp
It is possible to determine what strings correspond to subexpressions by
recursively applying the leftmost longest rule to each subexpression, but
only with the proviso that the overall match is leftmost longest. For
example, matching \\(ac*\\)c*d[ac]*\\1 against acdacaaa matches acdacaaa
(with \\1=a); simply matching the longest match for \\(ac*\\) would yield
\\1=ac, but the overall match would be smaller (acdac). Conceptually, the
implementation must examine every possible match and among those that yield
the leftmost longest total matches, pick the one that does the longest match
for the leftmost subexpression and so on. Note that this means that matching
by subexpressions is context-dependent: a subexpression within a larger RE
may match a different string from the one it would match as an independent RE,
and two instances of the same subexpression within the same larger RE may
match different lengths even in similar sequences of characters. For example,
in the ERE (a.*b)(a.*b), the two identical subexpressions would match four
and six characters, respectively, of accbaccccb.
.sp
When a multi-character collating element in a bracket expression
is involved, the longest sequence will be measured in
characters consumed from the string to be matched; that is, the collating
element counts not as one element, but as the number of characters it matches.
.in-0.5i
.PP
\f4BRE (ERE) matching a single character\fP
.PP
.in+0.5i
A BRE or ERE that matches either a single character or a single collating
element.
.in-0.5i
.PP
.in+0.5i
Only a BRE or ERE of this type that includes a bracket expression
can match a collating element.
.in-0.5i
.PP
.in+0.5i
The definition of \f2single\fP character has been expanded to include also
collating elements consisting of two or more characters; this expansion is
applicable only when a bracket expression is included in the BRE or ERE. An
example of such a collating element may be the Dutch ij, which collates as a
y. In some encodings, a ligature ``i with j'' exists as a character and
would represent a single-character collating element. In another encoding,
no such ligature exists, and the two-character sequence ij is defined as a
multi-character collating element. Outside brackets, the ij is treated as a
two-character RE and matches the same characters in a string. Historically,
a bracket expression only matched a single character. If, however, the
bracket expression defines, for example, a range that includes ij, then
this particular bracket expression will also match a sequence of the two
characters i and j in the string.
.in-0.5i
.PP
\f4BRE (ERE) matching multiple characters\fP
.PP
.in+0.5i
A BRE or ERE that matches a concatenation of single characters or collating
elements.
.in-0.5i
.PP
\f4invalid\fP
.PP
.in+0.5i
This section uses the term \f2invalid\fP for certain constructs or conditions.
Invalid REs will cause the utility or function using the RE to generate an
error condition. When \f2invalid\fP is not used, violations of the specified
syntax or semantics for REs produce undefined results: this may entail an
error, enabling an extended syntax for that RE, or using the construct in
error as literal characters to be matched. For example, the BRE construct
\\{1,2,3\\} does not comply with the grammar. A portable application
cannot rely on it producing an error nor matching the literal characters
\\{1,2,3\\}.
.in-0.5i
.SH "Regular Expression General Requirements"
.sp
The requirements in this section apply to both basic and extended regular
expressions.
.sp
The use of regular expressions is generally associated with text processing.
REs (BREs and EREs) operate on text strings; that is, zero or more characters
followed by an end-of-string delimiter (typically NUL). Some utilities
employing regular expressions limit the processing to lines; that is, zero
or more characters followed by a newline character. In the regular
expression processing described in this document, the newline character is
regarded as an ordinary character and both a period and a non-matching list
can match one. The individual man pages specify within the individual
descriptions of those standard utilities employing regular expressions
whether they permit matching of newline characters; if not stated otherwise,
the use of literal newline characters or any escape sequence equivalent
produces undefined results. Those utilities (like \f2grep\fP) that do not
allow newline characters to match are responsible for eliminating any newline
character from strings before matching against the RE. The \f2regcomp()\fP
function (see \f4regcomp(3G)\fP), however, can provide support for such
processing without violating the rules of this section.
.PP
The interfaces specified in this document set do not permit the inclusion of
a NUL character in an RE or in the string to be matched. If during the
operation of a standard utility a NUL is included in the text designated to
be matched, that NUL may designate the end of the text string for the purposes
of matching.
.PP
When a standard utility or function that uses regular expressions specifies
that pattern matching will be performed without regard to the case
(upper- or lower-) of either data or patterns, then when each character in
the string is matched against the pattern, not only the character, but also
its case counterpart (if any), will be matched. This definition of
case-insensitive processing is intended to allow matching of multi-character
collating elements as well as characters. For instance, as each character in
the string is matched using both its cases, the RE [[.Ch.]] when matched
against the string char, is in reality matched against ch, Ch, cH and CH.
.PP
The implementation will support any regular expression that does not exceed
256 bytes in length.
.SH "Basic Regular Expressions"
.sp
.PP
\f4BREs Matching a Single Character or Collating Element\fP
.PP
.in+0.5i
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.
.in-0.5i
.PP
\f4BRE Ordinary Characters\fP
.PP
.in+0.5i
An ordinary character is a BRE that matches itself: any character in the
supported character set, except for the BRE special characters listed in
\f4brespec\fP.
.sp
The interpretation of an ordinary character preceded by a backslash (\\) is
undefined, except for:
.sp
1. the characters ), (, { and }
.sp
2. the digits 1 to 9 inclusive
.sp
3. a character inside a bracket expression.
.in-0.5i
.PP
\f4BRE Special Characters\fP
.PP
.in+0.5i
A \f2BRE special character\fP 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:
.TP .5i
.B .[\e\\
The period, left-bracket and backslash is special except when used in a
bracket expression. An expression containing a \f4[\fP
that is not preceded by a backslash and is not part of a bracket expression
produces undefined results.
.TP
.B *
The asterisk is special except when used:
.sp
o in a bracket expression
.sp
o as the first character of an entire BRE (after an initial ^,
.in+0.4i
if any)
.in-0.4i
.sp
o as the first character of a subexpression (after an initial ^,
.in+0.4i
if any).
.in-0.4i
.TP .5i
.B "^"
The circumflex is special when used:
.sp
o as an anchor
.sp
o as the first character of a bracket expression.
.TP .5i
.B "$"
The dollar sign is special when used as an anchor.
.PP
.sp
\f4Periods in BREs\fP
.sp
.in+0.5i
A period (\f4.\fP), when used outside a bracket expression, is a BRE that
matches any character in the supported character set except NUL.
.in-0.5i
.sp
.PP
\f4RE Bracket Expression\fP
.sp
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.
.sp
The following rules and definitions apply to bracket expressions:
.sp
.TP .5i
1.
A \f2bracket expression\fP 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. 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:
.sp
.B ". * [ \e\\"
.sp
(period, asterisk, left-bracket and backslash, respectively) lose their
special meaning within a bracket expression.
.sp
The character sequences:
.sp
.B "[. [= [:"
.sp
(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.
.TP
2.
A \f2matching list\fP 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.
.TP
3.
A \f2non-matching list\fP 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.
.TP
4.
A \f2collating symbol\fP is a collating element enclosed within
bracket-period ([. .]) delimiters. Collating elements are defined as
described in \f4colltbl(1M)\fP. 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 recognised 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
(for example, see the symbol <HIGH> in the example collation definition
shown in \f4colltbl(1M)\fP), the symbol will be treated as an invalid
expression.
.TP
5.
An \f2equivalence class expression\fP represents the set of collating elements
belonging to an equivalence class, as described in \f4colltbl(1M)\fP.
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, agrave and
acircumflex belong to the same equivalence class, then [=a=]b],
[[=agrave=]b] and [[=acircumflex=]b] will each be equivalent to
[aagraveacircumflexb]. If the collating element does not belong to an
equivalence class, the equivalence class expression will be treated as a
\f2collating symbol\fP.
.TP
6.
A \f2character class expression\fP 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
recognised. A character class expression is expressed as a character class
name enclosed within bracket-colon ([: :]) delimiters.
.sp
The following character class expressions are supported in all locales:
.sp
.sp
The following character class expressions are supported in all locales:
.sp
.nf
.na
[:alnum:] [:cntrl:] [:lower:] [:space:]
[:alpha:] [:digit:] [:print:] [:upper:]
[:blank:] [:graph:] [:punct:] [:xdigit:]
.fi
.sp
In addition, character class expressions of the form:
.sp
.in+0.5i
[:name:]
.in-0.5i
.sp
are recognised in those locales where the \f2name\fP keyword has been given a
\f4charclass\fP definition in the LC_CTYPE category.
.TP
7.
A \f2range expression\fP 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 (\f4-\fP).
.sp
Range expressions must not be used in portable applications because their
behaviour 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 \f2aumlat\fP as a variant of a, while another defines it as a letter
following z, then the expression [\f2aumlat\fP-z] is valid in the first
language and invalid in the second.
.sp
In the following, all examples assume the collation sequence specified for
the POSIX locale, unless another collation sequence is specifically defined.
.sp
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 the description of \f2LC_COLLATE\fP in \f4locale(1)\fP) will not
be performed. For example, assuming that the character \f2eszet\fP is
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, \f2eszet\fP ot t.
.sp
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.
.sp
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.
.sp
If a bracket expression must specify both - and ], the ] must be placed
first (after the ^, if any) and the - last within the bracket expression.
.sp
\f4BREs Matching Multiple Characters\fP
.sp
The following rules can be used to construct BREs matching multiple
characters from BREs matching a single character:
.sp
.TP .5i
1.
The concatenation of BREs matches the concatenation of the strings matched
by each component of the BRE.
.TP
2.
A \f2subexpression\fP can be defined within a BRE by enclosing it between
the character pairs \\( and \\) . Such a subexpression matches whatever it
would have matched without the \\( and \\), except that anchoring within
subexpressions is optional behaviour. Subexpressions can
be arbitrarily nested.
.TP
3.
The \f2back-reference\fP expression \f2\\n\fP matches the same
(possibly empty) string of characters as was matched by a subexpression
enclosed between \\( and \\) preceding the \f2\\n\fP. The character \f2\fPn
must be a digit from 1 to 9 inclusive, \f2n\fPth subexpression (the one
that begins with the \f2n\fPth \\( and ends with the corresponding paired
\\)). The expression is invalid if less than \f2n\fP subexpressions precede
the \f2\\n\fP. 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 \f2a\fP. 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 subexpressions:
.in-0.5i
.sp
\\(\\(\\(ab\\)*c\\)*d\\)\\(ef\\)*\\(gh\\)\\{2\\}\\(ij\\)*\\(kl\\)*\\(mn\\)*\\(op\\)*\\(qr\\)*
.sp
.in+0.5i
.TP
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.
.TP
5.
When a BRE matching a single character, a subexpression or a back-reference
is followed by an \f2interval expression\fP of the format \\{\f2m\fP\\},
\\{\f2m\fP,\\} or \\{\f2m\fP,\f2n\fP\\}, together with that interval expression
it matches what repeated consecutive occurrences of the BRE would match. The
values of \f2m\fP and \f2n\fP will be decimal integers in the range
0 <= \f2m\fP <= \f2n\fP <= \f2RE_DUP_MAX\fP, where \f2m\fP specifies the
exact or minimum number of occurrences and \f2n\fP specifies the maximum
number of occurrences. The expression \\{\f2m\fP\\} matches exactly \f2m\fP
occurrences of the preceding BRE, \\{\f2m\fP,\\} matches at least \f2m\fP
occurrences and \\{\f2m,n\fP\\} matches any number of occurrences between
\f2m\fP and \f2n\fP, inclusive.
.sp
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.
.sp
.in-0.5i
The behaviour of multiple adjacent duplication symbols (\f4*\fP and intervals)
produces undefined results.
.sp
\f4BRE Precedence\fP
.sp
.in+0.5i
The order of precedence is as shown in the following table:
.sp
\f2BRE Precedence (from high to low)\fP
.sp
.nf
.in+0.5i
.I
collation-related bracket symbols [= =] [: :] [. .]
.sp
.I
escaped characters \\<special character>
.sp
.I
bracket expression []
.sp
.I
subexpressions/back-references \\(\\)\\n
.sp
.I
single-character-BRE duplication *\\{m,n\\}
.sp
.I
concatenation
.sp
.I
anchoring ^ $
.in-0.5i
.fi
.in-0.5i
.sp
\f4BRE Expression Anchoring\fP
.sp
.in+0.5i
A BRE can be limited to matching strings that begin or end a line; this is
called \f2anchoring\fP. The circumflex and dollar sign special characters
will be considered BRE anchors in the following contexts:
.sp
.TP .5i
1.
A circumflex (\f4^\fP) 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 (or optionally subexpression) 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. The BRE \\(^ab\\) may match the former
string. A portable BRE must escape a leading circumflex in a subexpression
to match a literal circumflex.
.TP
2.
A dollar sign (\f4$\fP) is an anchor when used as the last character of an
entire BRE. The implementation may treat a dollar sign as an anchor when
used as the last character of a subexpression. The dollar sign will anchor
the expression (or optionally subexpression) to the end of the string being
matched; the dollar sign can be said to match the end-of-string following
the last character.
.TP
3.
A BRE anchored by both \f4^\fP and \f4$\fP matches only an entire string.
For example, the BRE ^abcdef$ matches strings consisting only of abcdef.
.in-0.5i
.sp
\f4Extended Regular Expressions\fP
.sp
.in+0.5i
The \f2extended regular expression\fP (ERE) notation and construction rules
will apply to utilities defined as using extended regular expressions; any
exceptions to the following rules are noted in the descriptions of the
specific utilities using EREs.
.in-0.5i
.sp
\f4EREs Matching a Single Character or Collating Element\fP
.sp
.in+0.5i
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
\f2ERE matching a single character\fP enclosed in parentheses matches the
same as the ERE without parentheses would have matched.
.in-0.5i
.sp
\f4ERE Ordinary Characters\fP
.sp
.in+0.5i
An \f2ordinary character\fP 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 \f4erespec\fP. The interpretation of an
ordinary character preceded by a backslash (\f4\\\fP) is undefined.
.in-0.5i
.sp
\f4ERE Special Characters\fP
.sp
.in+0.5i
An \f2ERE special character\fP 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:
.sp
.TP .5i
.B ". [ \e\\ ("
The period, left-bracket, backslash and left-parenthesis are special except
when used in a bracket expression. Outside a bracket
expression, a left-parenthesis immediately followed by a right-parenthesis
produces undefined results.
.TP
.B ")"
The right-parenthesis is special when matched with a preceding
left-parenthesis, both outside a bracket expression.
.TP
.B "* + ? {"
The asterisk, plus-sign, question-mark and left-brace are special except
when used in a bracket expression. Any of the following
uses produce undefined results:
.sp
.in+0.5i
if these characters appear first in an ERE, or immediately following a
vertical-line, circumflex or left-parenthesis
.sp
if a left-brace is not part of a valid interval expression.
.in-0.5i
.TP .5i
.B "|"
The vertical-line is special except when used in a bracket expression.
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.
.TP
.B "^"
The circumflex is special when used:
.sp
.in+0.5i
as an anchor
.sp
as the first character of a bracket expression.
.in-0.5i
.TP
.B "$"
The dollar sign is special when used as an anchor.
.sp
.in-0.5i
\f4Periods in EREs\fP
.sp
.in+0.5i
A period (\f4.\fP), when used outside a bracket expression, is an ERE that
matches any character in the supported character set except NUL.
.in-0.5i
.sp
\f4EREs Matching Multiple Characters\fP
.sp
.in+0.5i
The following rules will be used to construct EREs matching multiple
characters from EREs matching a single character:
.in-0.5i
.TP .5i
1.
A \f2concatenation of EREs\fP 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.
.TP
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. And, [ab]+ and [ab][ab]* are equivalent.
.TP
3.
When an ERE matching a single character or an ERE enclosed in parentheses is
followed by the special character asterisk (\f4*\fP), 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.
.TP
4.
When an ERE matching a single character or an ERE enclosed in parentheses
is followed by the special character question-mark (\f4?\fP), 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.
.TP
5.
When an ERE matching a single character or an ERE enclosed in parentheses
is followed by an \f2interval expression\fP of the format {\f2m\fP},
{\f2m\fP,} or {\f2m\fP,\f2n\fP}, together with that interval expression it
matches what repeated consecutive occurrences of the ERE would match. The
values of \f2m\fP and \f2n\fP will be decimal integers in the range
0 <= \f2m\fP <= \f2n\fP <= \f2RE_DUP_MAX\fP, where \f2m\fP specifies the
exact or minimum number of occurrences and \f2n\fP specifies the maximum
number of occurrences. The expression {\f2m\fP} matches exactly \f2m\fP
occurrences of the preceding ERE, {\f2m\fP,} matches at least \f2m\fP
occurrences and {\f2m\fP,\f2n\fP} matches any number of occurrences between
\f2m\fP and \f2n\fP, 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.
.sp
.in-0.5i
The behaviour of multiple adjacent duplication symbols (\f4+, *, ?\fP and
intervals) produces undefined results.
.sp
\f4ERE Alternation\fP
.sp
.in+0.5i
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.
.in-0.5i
.sp
\f4ERE Precedence\fP
.sp
.in+0.5i
The order of precedence is as shown in the following table:
.sp
\f2BRE Precedence (from high to low)\fP
.sp
.nf
.in+0.5i
.I
collation-related bracket symbols [= =] [: :] [. .]
.sp
.I
escaped characters \\<special character>
.sp
.I
bracket expression []
.sp
.I
grouping ()
.sp
.I
single-character-ERE duplication *+?{m,n}
.sp
.I
concatenation
.sp
.I
anchoring ^ $
.sp
.I
alteration |
.in-0.5i
.fi
.in-0.5i
.sp
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).
.sp
\f4ERE Expression Anchoring\fP
.sp
.in+0.5i
An ERE can be limited to matching strings that begin or end a line; this is
called \f2anchoring\fP. The circumflex and dollar sign special characters
are considered ERE anchors when used anywhere outside a bracket expression.
This has the following effects:
.sp
.TP .5i
1.
A circumflex (\f4^\fP) 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 \f2a\fP prevents the expression ^b from
matching starting at the first character.
.TP
2.
A dollar sign (\f4$\fP) outside a bracket expression anchors the expression
or subexpression it ends to the end of a string; such an expression or
subexpression can match only a sequence ending at the last character of a
string. For example, the EREs ef$ 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 \f2f\fP prevents the expression e$ from matching
ending at the last character.
.in-0.5i
.sp
\f4Regular Expression Grammar\fP
.sp
.in+0.5i
Grammars describing the syntax of both basic and extended regular
expressions are presented in this section. The grammar takes precedence
over the text.
.in-0.5i
.sp
\f4BRE/ERE Grammar Lexical Conventions\fP
.sp
.in+0.5i
The lexical conventions for regular expressions are as described in this
section.
.sp
Except as noted, the longest possible token or delimiter beginning at a
given point will be recognised.
.sp
The following tokens will be processed (in addition to those string constants
shown in the grammar):
.sp
.TP 1.5i
.B COLL_ELEM
Any single-character collating element, unless it is a META_CHAR.
.TP
.B BACKREF
Applicable only to basic regular expressions. The character string
consisting of \f4\\\fP followed by a single-digit numeral, 1 to 9.
.TP
.B DUP_COUNT
Represents a numeric constant. It is an integer in the range 0 <=
\f2DUP_COUNT\fP <= \f2RE_DUP_MAX\fP. This token will only be recognised
when the context of the grammar requires it. At all other times, digits
not preceded by \f4\\\fP will be treated as ORD_CHAR.
.TP
.B META_CHAR
One of the characters:
.sp
.in+0.5i
\f4^\fP when found first in a bracket expression
.sp
\f4\-\fP when found anywhere but first (after an initial
.in+0.4i
\f4^\fP, if any) or last in a bracket expression, or as the ending
.br
range point in a range expression
.in-0.4i
.sp
\f4]\fP when found anywhere but first (after an initial
.in+0.4i
\f4^\fP, if any) in a bracket expression.
.in-0.4i
.in-0.5i
.TP
.B L_ANCHOR
Applicable only to basic regular expressions. The character \f4^\fP when it
appears as the first character of a basic regular expression and when not
QUOTED_CHAR. The \f4^\fP may be recognised as an anchor elsewhere.
.TP
.B ORD_CHAR
A character, other than one of the special characters in SPEC_CHAR.
.TP
.B QUOTED_CHAR
In a BRE, one of the character sequences:
.sp
\\^ \\. \\* \\[ \\$ \\\\
.sp
In an ERE, one of the character sequences:
.sp
\\^ \\. \\[ \\$ \\( \\) \\| \\* \\+ \\? \\{ \\\\
.TP
.B R_ANCHOR
(Applicable only to basic regular expressions.) The character \f4$\fP when
it appears as the last character of a basic regular expression and when not
QUOTED_CHAR. The \f4$\fP may be recognised as an anchor elsewhere.
.TP
.B SPEC_CHAR
For basic regular expressions, will be one of the following special
characters:
.sp
\f4.\fP anywhere outside bracket expressions
.sp
\f4\\\f1 anywhere outside bracket expressions
.sp
\f4[\fP anywhere outside bracket expressions
.sp
\f4^\fP when used as an anchor or when
.in+0.4i
first in a bracket expression
.in-0.4i
.sp
\f4$\fP when used as an anchor
.sp
\f4*\fP anywhere except: first in an entire RE;
.in+0.4i
anywhere in a bracket expression; directly
.br
following \\(; directly following an
.br
anchoring \f4^\fP.
.in-0.4i
.sp
For extended regular expressions, will be one of the following special
characters found anywhere outside bracket expressions:
.sp
^ . [ $ ( ) | * + ? { \\
.sp
(The close-parenthesis is considered special in this context only if matched
with a preceding open-parenthesis.)
.in-0.5i
.sp
.SH "RE and Bracket Expression Grammar"
.sp
This section presents the grammar for basic regular expressions, including
the bracket expression grammar that is common to both BREs and EREs.
.sp
.nf
%token ORD_CHAR QUOTED_CHAR DUP_COUNT
%token BACKREF L_ANCHOR R_ANCHOR
%token Back_open_paren Back_close_paren
/* '\\(' '\\)' */
%token Back_open_brace Back_close_brace
/* '\\{' '\\}' */
/* The following tokens are for the Bracket Expression
grammar common to both REs and EREs. */
%token COLL_ELEM META_CHAR
%token Open_equal Equal_close Open_dot Dot_close Open_colon Colon_close
/* '[=' '=]' '[.' '.]' '[:' ':]' */
%token class_name
/* class_name is a keyword to the LC_CTYPE locale category */
/* (representing a character class) in the current locale */
/* and is only recognised between [: and :] */
%start basic_reg_exp
%%
/* --------------------------------------------
Basic Regular Expression
--------------------------------------------
*/
basic_reg_exp : RE_expression
| L_ANCHOR
| R_ANCHOR
| L_ANCHOR R_ANCHOR
| L_ANCHOR RE_expression
| RE_expression R_ANCHOR
| L_ANCHOR RE_expression R_ANCHOR
;
RE_expression : simple_RE
| RE_expression simple_RE
;
simple_RE : nondupl_RE
| nondupl_RE RE_dupl_symbol
;
nondupl_RE : one_character_RE
| Back_open_paren RE_expression Back_close_paren
| Back_open_paren Back_close_paren
| BACKREF
;
one_character_RE : ORD_CHAR
| QUOTED_CHAR
| '.'
| bracket_expression
;
RE_dupl_symbol : '*'
| Back_open_brace DUP_COUNT Back_close_brace
| Back_open_brace DUP_COUNT ',' Back_close_brace
| Back_open_brace DUP_COUNT ',' DUP_COUNT Back_close_brace
;
/* --------------------------------------------
Bracket Expression
-------------------------------------------
*/
bracket_expression : '[' matching_list ']'
| '[' nonmatching_list ']'
;
matching_list : bracket_list
;
nonmatching_list : '^' bracket_list
;
bracket_list : follow_list
| follow_list '-'
;
follow_list : expression_term
| follow_list expression_term
;
expression_term : single_expression
| range_expression
;
single_expression : end_range
| character_class
| equivalence_class
;
range_expression : start_range end_range
| start_range '-'
;
start_range : end_range '-'
;
end_range : COLL_ELEM
| collating_symbol
;
collating_symbol : Open_dot COLL_ELEM Dot_close
| Open_dot META_CHAR Dot_close
;
equivalence_class : Open_equal COLL_ELEM Equal_close
;
character_class : Open_colon class_name Colon_close
;
.fi
.sp
The BRE grammar does not permit L_ANCHOR or R_ANCHOR inside \\( and \\)
(which implies that ^ and $ are ordinary characters).
.sp
.SH "ERE Grammar"
.sp
This section presents the grammar for extended regular expressions, excluding
the bracket expression grammar.
.sp
\f4Note:\fP The bracket expression grammar and the associated \f4%token\fP
.in+0.7i
lines are identical between BREs and EREs. It has been omitted
.br
from the ERE section to avoid unnecessary editorial duplication.
.in-0.7i
.sp
.nf
%token ORD_CHAR QUOTED_CHAR DUP_COUNT
%start extended_reg_exp
%%
/* --------------------------------------------
Extended Regular Expression
--------------------------------------------
*/
extended_reg_exp : ERE_branch
| extended_reg_exp ' | ' ERE_branch
;
ERE_branch : ERE_expression
| ERE_branch ERE_expression
;
ERE_expression : one_character_ERE
| '^'
| '$'
| '(' extended_reg_exp ')'
| ERE_expression ERE_dupl_symbol
;
one_character_ERE : ORD_CHAR
| QUOTED_CHAR
| '.'
| bracket_expression
;
ERE_dupl_symbol : '*'
| '+'
| '?'
| '{' DUP_COUNT '}'
| '{' DUP_COUNT ',' '}'
| '{' DUP_COUNT ',' DUP_COUNT '}'
;
.fi
.sp
The ERE grammar does not permit several constructs that previous sections
specify as having undefined results:
.sp
o ORD_CHAR preceded by \\
.sp
o one or more ERE_dupl_symbols appearing first in an ERE,
.in+0.4i
or immediately following \f4|\fP, \f4^\fP or \f4(\fP
.in-0.4i
.sp
o \f4{\fP not part of a valid ERE_dupl_symbol
.sp
o \f4|\fP appearing first or last in an ERE,
.in+0.4i
or immediately following \f4|\fP or
.br
\f4(\fP, or immediately preceding \f4)\fP.
.in-0.4i
.sp
Implementations are permitted to extend the language to allow these. Portable
applications cannot use such constructs.
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