xbd_chap09.html

IEEE 1003.1-2003, Single Unix Specification v3

<li>
<p>The set of single-character collating elements whose characters belong to the character class, as defined in the <i>LC_CTYPE</i>
category in the current locale.</p>
</li>

<li>
<p>An unspecified set of multi-character collating elements.</p>
</li>
</ol>

<p>All character classes specified in the current locale shall be recognized. A character class expression is expressed as a
character class name enclosed within bracket-colon ( <tt>"[:"</tt> and <tt>":]"</tt> ) delimiters.</p>

<p>The following character class expressions shall be supported in all locales:</p>

<blockquote>
<pre>
<tt>[:alnum:]   [:cntrl:]   [:lower:]   [:space:]
[:alpha:]   [:digit:]   [:print:]   [:upper:]
[:blank:]   [:graph:]   [:punct:]   [:xdigit:]
</tt>
</pre>
</blockquote>

<p>In addition, character class expressions of the form:</p>

<blockquote>
<pre>
<tt>[:</tt><i>name</i><tt>:]
</tt>
</pre>
</blockquote>

<p>are recognized in those locales where the <i>name</i> keyword has been given a <b>charclass</b> definition in the
<i>LC_CTYPE</i> category.</p>
</li>

<li>
<p>In the POSIX locale, a range expression represents the set of collating elements that fall between two elements in the collation
sequence, inclusive. In other locales, a range expression has unspecified behavior: strictly conforming applications shall not rely
on whether the range expression is valid, or on the set of collating elements matched. A range expression shall be expressed as the
starting point and the ending point separated by a hyphen ( <tt>'-'</tt> ).</p>

<p>In the following, all examples assume the POSIX locale.</p>

<p>The starting range point and the ending range point shall 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. If the represented set of collating elements is empty, it is
unspecified whether the expression matches nothing, or is treated as invalid.</p>

<p>The interpretation of range expressions where the ending range point is also the starting range point of a subsequent range
expression (for example, <tt>"[a-m-o]"</tt> ) is undefined.</p>

<p>The hyphen character shall be treated as itself if it occurs first (after an initial <tt>'^'</tt> , if any) or last in the list,
or as an ending range point in a range expression. As examples, the expressions <tt>"[-ac]"</tt> and <tt>"[ac-]"</tt> are
equivalent and match any of the characters <tt>'a'</tt> , <tt>'c'</tt> , or <tt>'-'</tt> ; <tt>"[^-ac]"</tt> and <tt>"[^ac-]"</tt>
are equivalent and match any characters except <tt>'a'</tt> , <tt>'c'</tt> , or <tt>'-'</tt> ; the expression <tt>"[%--]"</tt>
matches any of the characters between <tt>'%'</tt> and <tt>'-'</tt> inclusive; the expression <tt>"[--@]"</tt> matches any of the
characters between <tt>'-'</tt> and <tt>'@'</tt> inclusive; and the expression <tt>"[a--@]"</tt> is either invalid or equivalent to
<tt>'@'</tt> , because the letter <tt>'a'</tt> follows the symbol <tt>'-'</tt> in the POSIX locale. To use a hyphen as the starting
range point, it shall either come first in the bracket expression or be specified as a collating symbol; for example,
<tt>"[][.-.]-0]"</tt> , which matches either a right bracket or any character or collating element that collates between hyphen and
0, inclusive.</p>

<p>If a bracket expression specifies both <tt>'-'</tt> and <tt>']'</tt> , the <tt>']'</tt> shall be placed first (after the
<tt>'^'</tt> , if any) and the <tt>'-'</tt> last within the bracket expression.</p>
</li>
</ol>

<h4><a name="tag_09_03_06"></a>BREs Matching Multiple Characters</h4>

<p>The following rules can be used to construct BREs matching multiple characters from BREs matching a single character:</p>

<ol>
<li>
<p>The concatenation of BREs shall match the concatenation of the strings matched by each component of the BRE.</p>
</li>

<li>
<p>A subexpression can be defined within a BRE by enclosing it between the character pairs <tt>"\("</tt> and <tt>"\)"</tt> . Such a
subexpression shall match whatever it would have matched without the <tt>"\("</tt> and <tt>"\)"</tt> , except that anchoring within
subexpressions is optional behavior; see <a href="#tag_09_03_08">BRE Expression Anchoring</a> . Subexpressions can be arbitrarily
nested.</p>
</li>

<li>
<p>The back-reference expression <tt>'\n'</tt> shall match the same (possibly empty) string of characters as was matched by a
subexpression enclosed between <tt>"\("</tt> and <tt>"\)"</tt> preceding the <tt>'\n'</tt> . The character <tt>'n'</tt> shall be a
digit from 1 through 9, specifying the <i>n</i>th subexpression (the one that begins with the <i>n</i>th <tt>"\("</tt> from the
beginning of the pattern and ends with the corresponding paired <tt>"\)"</tt> ). The expression is invalid if less than <i>n</i>
subexpressions precede the <tt>'\n'</tt> . For example, the expression <tt>"\(.*\)\1$"</tt> matches a line consisting of two
adjacent appearances of the same string, and the expression <tt>"\(a\)*\1"</tt> fails to match <tt>'a'</tt> . When the referenced
subexpression matched more than one string, the back-referenced expression shall refer to the last matched string. If the
subexpression referenced by the back-reference matches more than one string because of an asterisk ( <tt>'*'</tt> ) or an interval
expression (see item (5)), the back-reference shall match the last (rightmost) of these strings.</p>
</li>

<li>
<p>When a BRE matching a single character, a subexpression, or a back-reference is followed by the special character asterisk (
<tt>'*'</tt> ), together with that asterisk it shall match what zero or more consecutive occurrences of the BRE would match. For
example, <tt>"[ab]*"</tt> and <tt>"[ab][ab]"</tt> are equivalent when matching the string <tt>"ab"</tt> .</p>
</li>

<li>
<p>When a BRE matching a single character, a subexpression, or a back-reference is followed by an interval expression of the format
<tt>"\{m\}"</tt> , <tt>"\{m,\}"</tt> , or <tt>"\{m,n\}"</tt> , together with that interval expression it shall match what repeated
consecutive occurrences of the BRE would match. The values of <i>m</i> and <i>n</i> are 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 <tt>"\{m\}"</tt> shall match exactly <i>m</i> occurrences of the
preceding BRE, <tt>"\{m,\}"</tt> shall match at least <i>m</i> occurrences, and <tt>"\{m,n\}"</tt> shall match any number of
occurrences between <i>m</i> and <i>n</i>, inclusive.</p>

<p>For example, in the string <tt>"abababccccccd"</tt> the BRE <tt>"c\{3\}"</tt> is matched by characters seven to nine, the BRE
<tt>"\(ab\)\{4,\}"</tt> is not matched at all, and the BRE <tt>"c\{1,3\}d"</tt> is matched by characters ten to thirteen.</p>
</li>
</ol>

<p>The behavior of multiple adjacent duplication symbols ( <tt>'*'</tt> and intervals) produces undefined results.</p>

<p>A subexpression repeated by an asterisk ( <tt>'*'</tt> ) or an interval expression shall not match a null expression unless this
is the only match for the repetition or it is necessary to satisfy the exact or minimum number of occurrences for the interval
expression.</p>

<h4><a name="tag_09_03_07"></a>BRE Precedence</h4>

<p>The order of precedence shall be as shown in the following table:</p>

<center>
<table border="1" cellpadding="3" align="center">
<tr valign="top">
<th colspan="2" align="center">
<p class="tent"><b>BRE Precedence (from high to low)</b></p>
</th>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Collation-related bracket symbols</p>
</td>
<td align="left">
<p class="tent">[==] [::] [..]</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Escaped characters</p>
</td>
<td align="left">
<p class="tent">\&lt;special character&gt;</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Bracket expression</p>
</td>
<td align="left">
<p class="tent">[]</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Subexpressions/back-references</p>
</td>
<td align="left">
<p class="tent">\(\) \n</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Single-character-BRE duplication</p>
</td>
<td align="left">
<p class="tent">* \{m,n\}</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Concatenation</p>
</td>
<td align="left">
<p class="tent">&nbsp;</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent">Anchoring</p>
</td>
<td align="left">
<p class="tent">^ $</p>
</td>
</tr>
</table>
</center>

<h4><a name="tag_09_03_08"></a>BRE Expression Anchoring</h4>

<p>A BRE can be limited to matching strings that begin or end a line; this is called &quot;anchoring&quot;. The circumflex and dollar sign
special characters shall be considered BRE anchors in the following contexts:</p>

<ol>
<li>
<p>A circumflex ( <tt>'^'</tt> ) shall be an anchor when used as the first character of an entire BRE. The implementation may treat
the circumflex as an anchor when used as the first character of a subexpression. The circumflex shall anchor the expression (or
optionally subexpression) to the beginning of a string; only sequences starting at the first character of a string shall be matched
by the BRE. For example, the BRE <tt>"^ab"</tt> matches <tt>"ab"</tt> in the string <tt>"abcdef"</tt> , but fails to match in the
string <tt>"cdefab"</tt> . The BRE <tt>"\(^ab\)"</tt> may match the former string. A portable BRE shall escape a leading circumflex
in a subexpression to match a literal circumflex.</p>
</li>

<li>
<p>A dollar sign ( <tt>'$'</tt> ) shall be 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 shall 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>

<li>
<p>A BRE anchored by both <tt>'^'</tt> and <tt>'$'</tt> shall match only an entire string. For example, the BRE <tt>"^abcdef$"</tt>
matches strings consisting only of <tt>"abcdef"</tt> .</p>
</li>
</ol>

<h3><a name="tag_09_04"></a>Extended Regular Expressions</h3>

<p>The extended regular expression (ERE) notation and construction rules shall 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.</p>

<h4><a name="tag_09_04_01"></a>EREs Matching a Single Character or Collating Element</h4>

<p>An ERE ordinary character, a special character preceded by a backslash, or a period shall match a single character. A bracket
expression shall match a single character or a single collating element. An ERE matching a single character enclosed in parentheses
shall match the same as the ERE without parentheses would have matched.</p>

<h4><a name="tag_09_04_02"></a>ERE Ordinary Characters</h4>

<p>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 <a href="#tag_09_04_03">ERE Special Characters</a> . The interpretation of an
ordinary character preceded by a backslash ( <tt>'\'</tt> ) is undefined.</p>

<h4><a name="tag_09_04_03"></a>ERE Special Characters</h4>

<p>An ERE special character has special properties in certain contexts. Outside those contexts, or when preceded by a backslash,
such a character shall be an ERE that matches the special character itself. The extended regular expression special characters and
the contexts in which they shall have their special meaning are as follows:</p>

<dl compact>
<dt><tt>.[\(</tt></dt>

<dd>The period, left-bracket, backslash, and left-parenthesis shall be special except when used in a bracket expression (see <a
href="#tag_09_03_05">RE Bracket Expression</a> ). Outside a bracket expression, a left-parenthesis immediately followed by a
right-parenthesis produces undefined results.</dd>

<dt><tt>)</tt></dt>

<dd>The right-parenthesis shall be special when matched with a preceding left-parenthesis, both outside a bracket expression.</dd>

<dt><tt>*+?{</tt></dt>

<dd>The asterisk, plus-sign, question-mark, and left-brace shall be special except when used in a bracket expression (see <a href=
"#tag_09_03_05">RE Bracket Expression</a> ). Any of the following uses produce undefined results: 

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

<li>
<p>If a left-brace is not part of a valid interval expression (see <a href="#tag_09_04_06">EREs Matching Multiple Characters</a>
)</p>
</li>
</ul>
</dd>