re.html

unix 下的C开发手册,还用详细的例程。

<tr valign=top><td align=left>subexpressions/back-references
<td align=left>\( \) \<i>n</i>
<tr valign=top><td align=left>single-character-BRE duplication
<td align=left>* \{<i>m</i>,<i>n</i>\}
<tr valign=top><td align=left>concatenation
<td align=left>&nbsp;
<tr valign=top><td align=left>anchoring
<td align=left>^  $
</table>
</pre>
<h4><a name = "tag_007_003_008">&nbsp;</a>BRE Expression Anchoring</h4>
<xref type="3" name="breanc"></xref>
A BRE can be limited to matching strings that begin or end a line;
this is called <i>anchoring</i>.
The circumflex and dollar sign special characters
will be considered BRE anchors in the following contexts:
<ol>
<p>
<li>
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 (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.
<p>
<li>
A dollar sign
($)
is an anchor when used as the last character of an entire BRE.
The implementation may treat a dollar sign as an anchor
when used as the last character of a subexpression.
The dollar sign will anchor the
expression (or optionally subexpression)
to the end of the string being
matched; the dollar sign can be said to
match the end-of-string following the last character.
<p>
<li>
A BRE anchored by both "^" and "$" matches only an entire string.
For example, the BRE
^abcdef$
matches strings consisting
only of
abcdef.
<p>
</ol>
<h3><a name = "tag_007_004">&nbsp;</a>Extended Regular Expressions</h3>
<xref type="2" name="ere"></xref>
The
<i>extended regular expression</i>
(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.
<h4><a name = "tag_007_004_001">&nbsp;</a>EREs Matching a Single Character or Collating Element</h4>
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 <i>ERE matching a single character</i>
enclosed in parentheses matches the same as the
ERE without parentheses would have matched.
<h4><a name = "tag_007_004_002">&nbsp;</a>ERE Ordinary Characters</h4>
An
<i>ordinary character</i>
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
<xref href=erespec><a href="#tag_007_004_003">
ERE Special Characters
</a></xref>.
The interpretation of an ordinary character
preceded by a backslash
(\)
is undefined.
<h4><a name = "tag_007_004_003">&nbsp;</a>ERE Special Characters</h4>
<xref type="3" name="erespec"></xref>
An
<i>ERE special character</i>
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:
<dl compact>

<dt>. [ \ (<dd>The period, left-bracket, backslash and left-parenthesis
are special except when used in a
bracket expression (see
<xref href=rebrack><a href="#tag_007_003_005">
RE Bracket Expression
</a></xref>).
Outside a bracket expression, a left-parenthesis
immediately followed by a right-parenthesis produces undefined results.

<dt>)<dd>The right-parenthesis is special when matched with a
preceding left-parenthesis, both outside a bracket expression.

<dt>* + ? {<dd>The asterisk, plus-sign, question-mark and left-brace
are special except when used in a bracket expression (see
<xref href=rebrack><a href="#tag_007_003_005">
RE Bracket Expression
</a></xref>).
Any of the following uses produce undefined results:
<ul>

<li>
if these characters appear first in an ERE,
or immediately following a vertical-line, circumflex or left-parenthesis

<li>
if a left-brace is not part of a valid interval expression.

</ul>

<dt>|<dd>The vertical-line is special except when used in a bracket expression (see
<xref href=rebrack><a href="#tag_007_003_005">
RE Bracket Expression
</a></xref>).
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.

<dt>^<dd>The circumflex is special when used:
<ul>

<li>
as an anchor
(see
<xref href=ereanc><a href="#tag_007_004_009">
ERE Expression Anchoring
</a></xref>)

<li>
as the first character of a bracket expression
(see
<xref href=rebrack><a href="#tag_007_003_005">
RE Bracket Expression
</a></xref>).

</ul>

<dt>$<dd>The dollar sign is special when used as an anchor.

</dl>
<h4><a name = "tag_007_004_004">&nbsp;</a>Periods in EREs</h4>
A period
(.),
when used outside a bracket expression,
is an ERE that matches any
character in the supported character set except NUL.
<h4><a name = "tag_007_004_005">&nbsp;</a>ERE Bracket Expression</h4>
The rules for ERE Bracket Expressions are the same as for
Basic Regular Expressions; see
<xref href=rebrack><a href="#tag_007_003_005">
RE Bracket Expression
</a></xref>.
<h4><a name = "tag_007_004_006">&nbsp;</a>EREs Matching Multiple Characters</h4>
<p>
The following rules will be used to construct
EREs matching multiple characters
from EREs matching a single character:
<ol>
<p>
<li>
A <i>concatenation of EREs</i> 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.
<p>
<li>
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.
<p>
<li>
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.
<p>
<li>
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.
<p>
<li>
When an ERE matching a single character or an
ERE enclosed in parentheses is followed by an
<i>interval expression</i>
of the format
{<i>m</i>}, {<i>m</i>,} or
{<i>m</i>,<i>n</i>},
together with that interval expression
it matches
what repeated consecutive occurrences of the ERE would match.
The values of <i>m</i> and <i>n</i> will be decimal integers in the range
0
&lt;=
<i>m</i>
&lt;=
<i>n</i>
&lt;=
{RE_DUP_MAX},
where <i>m</i> specifies the exact or minimum number of
occurrences and <i>n</i> specifies the maximum number of occurrences.
The expression {<i>m</i>} matches exactly <i>m</i>
occurrences of the preceding
ERE,
{<i>m</i>,} matches at least <i>m</i> occurrences and
{<i>m</i>,<i>n</i>} matches any number of occurrences between
<i>m</i> and <i>n</i>, inclusive.
<p>
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.
<p>
</ol>
<p>
The behaviour of multiple adjacent duplication symbols
(+,
*,
?
and intervals) produces undefined results.
<h4><a name = "tag_007_004_007">&nbsp;</a>ERE Alternation</h4>
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.
<h4><a name = "tag_007_004_008">&nbsp;</a>ERE Precedence</h4>
The order of precedence will be as shown in the following table:
<pre>
<table  bordercolor=#000000 border=1 align=center><tr valign=top><th colspan=2 align=center><b>ERE Precedence (from high to low)</b>
<tr valign=top><td align=left>collation-related bracket symbols
<td align=left>[= =]  [: :]  [. .]
<tr valign=top><td align=left>escaped characters
<td align=left>\&lt;<i>special character</i>&gt;
<tr valign=top><td align=left>bracket expression
<td align=left>[ ]
<tr valign=top><td align=left>grouping
<td align=left>( )
<tr valign=top><td align=left>single-character-ERE duplication
<td align=left>* + ? {<i>m</i>,<i>n</i>}
<tr valign=top><td align=left>concatenation
<td align=left>&nbsp;
<tr valign=top><td align=left>anchoring
<td align=left>^  $
<tr valign=top><td align=left>alternation
<td align=left>|
</table>
</pre>
<p>
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).
<h4><a name = "tag_007_004_009">&nbsp;</a>ERE Expression Anchoring</h4>
<xref type="3" name="ereanc"></xref>
An ERE can be limited to matching strings that begin or end a line;
this is called <i>anchoring</i>.
The circumflex and dollar sign special characters
are considered ERE anchors
when used anywhere outside a bracket expression.
This has the following effects:
<ol>
<p>
<li>
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,