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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.
<p>
<li>
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.
<p>
</ol>
<h3><a name = "tag_007_005">&nbsp;</a>Regular Expression Grammar</h3>
<xref type="2" name="regram"></xref>
Grammars describing the syntax of both
basic and extended regular expressions
are presented in this section.
The grammar takes precedence over the text.
See
the <b>XCU</b> specification, <b>Section 1.8</b>, <b>Grammar Conventions</b>.
<h4><a name = "tag_007_005_001">&nbsp;</a>BRE/ERE Grammar Lexical Conventions</h4>
The lexical conventions for regular expressions
are as described in this section.
<p>
Except as noted, the longest possible
token or delimiter beginning at a given point will be recognised.
<p>
The following tokens will be processed (in addition
to those string constants shown in the grammar):
<dl compact>

<dt>COLL_ELEM<dd>Any single-character collating element, unless it is a
META_CHAR.

<dt>BACKREF<dd>Applicable only to basic regular expressions.
The character string consisting of "\" followed by a single-digit numeral,
1 to 9.

<dt>DUP_COUNT<dd>Represents a numeric constant.
It is an integer in the range
0 &lt;=
DUP_COUNT
&lt;=
{RE_DUP_MAX}.
This token will only be recognised when the
context of the grammar requires it.
At all other times, digits not preceded by "\" will be treated as ORD_CHAR.

<dt>META_CHAR<dd>One of the characters:
<dl compact>

<dt>^<dd>when found first in a bracket expression

<dt>-<dd>when found anywhere but first (after an initial "^", if any)
or last in a bracket expression, or as the ending range point
in a range expression

<dt>]<dd>when found anywhere but first (after an initial "^" if any) in
a bracket expression.

</dl>
<p>
<dt>L_ANCHOR<dd>Applicable only to basic regular expressions.
The character "^" when it appears as the first character of
a basic regular expression and when not QUOTED_CHAR.
The "^" may be recognised as an anchor elsewhere; see
<xref href=breanc><a href="#tag_007_003_008">
BRE Expression Anchoring
</a></xref>.
<p>
<dt>ORD_CHAR<dd>A character, other than one of the special characters in SPEC_CHAR.
<p>
<dt>QUOTED_CHAR<dd>In a BRE, one of the character sequences:
<pre>
<dl compact><dt> <dd>
\^      \.      \*      \[      \$      \\
</dl>
</pre>
<p>
In an ERE, one of the character sequences:
<pre>
<dl compact><dt> <dd>
\^      \.      \[      \$      \(      \)    \|
\*      \+     \?      \{      \\
</dl>
</pre>
<p>
<dt>R_ANCHOR<dd>(Applicable only to basic regular expressions.)
The character "$" when it appears as the last character of a
basic regular expression and when not QUOTED_CHAR.
The "$" may be recognised as an anchor elsewhere; see
<xref href=breanc><a href="#tag_007_003_008">
BRE Expression Anchoring
</a></xref>.
<p>
<dt>SPEC_CHAR<dd>For basic regular expressions,
will be one of the following special characters:
<dl compact>

<dt>.<dd>anywhere outside bracket expressions

<dt>\<dd>anywhere outside bracket expressions

<dt>[<dd>anywhere outside bracket expressions

<dt>^<dd>when used as an anchor (see
<xref href=breanc><a href="#tag_007_003_008">
BRE Expression Anchoring
</a></xref>)
or when first in a bracket expression

<dt>$<dd>when used as an anchor

<dt>*<dd>anywhere except:
first in an entire RE;
anywhere in a bracket expression;
directly following
\(;
directly following an anchoring "^".

</dl>
<p>
For extended regular expressions,
will be one of the following special characters found anywhere
outside bracket expressions:
<pre>
<dl compact><dt> <dd>
^    .    [    $    (    )    |    *    +    ?    {    \
</dl>
</pre>
<p>
(The close-parenthesis is considered special in this context
only if matched with a preceding open-parenthesis.)
<p>
</dl>
<h4><a name = "tag_007_005_002">&nbsp;</a>RE and Bracket Expression Grammar</h4>
This section presents the grammar for basic regular expressions,
including the bracket expression grammar that is
common to both BREs and EREs.
<code>
<pre>
%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
               ;
</code>
</pre>
<p>
The BRE grammar does not permit L_ANCHOR or R_ANCHOR inside \(
and \) (which implies that ^ and $ are ordinary characters).
This reflects the semantic
limits on the application, as noted in
<xref href=breanc><a href="#tag_007_003_008">
BRE Expression Anchoring
</a></xref>.
Implementations are permitted to extend the language to interpret "^"
and "$" as anchors in these locations, and as
such, portable applications cannot use unescaped "^" and "$"
in positions inside
\(
and
\)
that might be interpreted as anchors.
<h4><a name = "tag_007_005_003">&nbsp;</a>ERE Grammar</h4>
This section presents the grammar for extended regular expressions,
excluding the bracket expression grammar.
<dl><dt><b>Note:</b>
<dd>The bracket expression grammar and the associated
<b>%token</b>
lines are identical between BREs and EREs.
It has been omitted from the ERE section to avoid unnecessary
editorial duplication.
</dl>
<code>
<pre>
%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 '}'
                   ;
</code>
</pre>
<p>
The ERE grammar does not permit several constructs that
previous sections specify as having undefined results:
<ul>
<p>
<li>
ORD_CHAR preceded by "\"
<p>
<li>
one or more ERE_dupl_symbols appearing first in an ERE, or
immediately following "|", "^" or "("
<p>
<li>
"{" not part of a valid ERE_dupl_symbol
<p>
<li>
"|" appearing first or last in an ERE, or immediately following
"|" or "(", or immediately preceding ")".
<p>
</ul>
<p>
Implementations are permitted to extend the language to allow these.
Portable applications cannot use such constructs.


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