2005-01-13 17:06:21 +00:00
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<title>Boost.Regex: POSIX-Basic Regular Expression Syntax</title>
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<td vAlign="top" width="300">
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<h3><A href="../../../index.htm"><IMG height="86" alt="C++ Boost" src="../../../boost.png" width="277" border="0"></A></h3>
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</td>
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<TD width="353">
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<H1 align="center">Boost.Regex</H1>
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<H2 align="center">POSIX Basic Regular Expression Syntax</H2>
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</TD>
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<td width="50">
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<h3><A href="index.html"><IMG height="45" alt="Boost.Regex Index" src="uarrow.gif" width="43" border="0"></A></h3>
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</P>
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<HR>
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<H3>Contents</H3>
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<dl class="index">
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<dt><A href="#synopsis">Synopsis</A> <dt><A href="#Basic">POSIX Basic Syntax</A> <dt><A href="#variations">
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Variations</A>
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<dd>
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<dl>
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<dt><A href="#grep">Grep</A> <dt><A href="#emacs">Emacs</A></dt>
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</dl>
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<dt><A href="#options">Options</A> <dt><A href="#refs">References</A></dt>
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</dl>
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<H3><A name="synopsis"></A>Synopsis</H3>
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<P>The POSIX-Basic regular expression syntax is used by the Unix utility <EM>sed</EM>,
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and variations are used by <EM>grep</EM> and <EM>emacs</EM>. You can
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construct POSIX basic regular expressions in Boost.Regex by passing the flag <EM>basic</EM>
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to the regex constructor, for example:</P>
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<PRE>// e1 is a case sensitive POSIX-Basic expression:
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boost::regex e1(my_expression, boost::regex::basic);
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// e2 a case insensitive POSIX-Basic expression:
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boost::regex e2(my_expression, boost::regex::basic|boost::regex::icase);</PRE>
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<H3>POSIX Basic Syntax<A name="Basic"></A></H3>
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<P>In POSIX-Basic regular expressions, all characters are match themselves except
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for the following special characters:</P>
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<PRE>.[\*^$</PRE>
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<H4>Wildcard:</H4>
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<P>The single character '.' when used outside of a character set will match any
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single character except:</P>
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<P>The NULL character when the flag <EM>match_no_dot_null</EM> is passed to the
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matching algorithms.</P>
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<P>The newline character when the flag <EM>match_not_dot_newline</EM> is passed to
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the matching algorithms.</P>
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<H4>Anchors:</H4>
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<P>A '^' character shall match the start of a line when used as the first
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character of an expression, or the first character of a sub-expression.</P>
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<P>A '$' character shall match the end of a line when used as the last character
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of an expression, or the last character of a sub-expression.</P>
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<H4>Marked sub-expressions:</H4>
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<P>A section beginning \( and ending \) acts as a marked sub-expression.
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Whatever matched the sub-expression is split out in a separate field by the
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matching algorithms. Marked sub-expressions can also repeated, or
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referred-to by a back-reference.</P>
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<H4>Repeats:</H4>
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<P>Any atom (a single character, a marked sub-expression, or a character class)
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can be repeated with the * operator.</P>
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<P>For example a* will match any number of letter a's repeated zero or more times
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(an atom repeated zero times matches an empty string), so the expression a*b
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will match any of the following:</P>
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<PRE>b
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ab
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aaaaaaaab</PRE>
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<P>An atom can also be repeated with a bounded repeat:</P>
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<P>a\{n\} Matches 'a' repeated exactly <EM>n</EM> times.</P>
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<P>a\{n,\} Matches 'a' repeated <EM>n</EM> or more times.</P>
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<P>a\{n, m\} Matches 'a' repeated between <EM>n</EM> and <EM>m</EM> times
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inclusive.</P>
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<P>For example:</P>
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<PRE>^a\{2,3\}$</PRE>
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<P>Will match either of:</P>
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<PRE>aa
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aaa</PRE>
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<P>But neither of:</P>
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<PRE>a
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aaaa</PRE>
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<P>It is an error to use a repeat operator, if the preceding construct can not be
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repeated, for example:</P>
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<PRE>a\(*\)</PRE>
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<P>Will raise an error, as there is nothing for the * operator to be applied to.</P>
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<H4>Back references:</H4>
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<P>An escape character followed by a digit <EM>n</EM>, where <EM>n </EM>is in the
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range 1-9, matches the same string that was matched by sub-expression <EM>n</EM>.
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For example the expression:</P>
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<PRE>^\(a*\).*\1$</PRE>
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<P>Will match the string:</P>
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<PRE>aaabbaaa</PRE>
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<P>But not the string:</P>
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<PRE>aaabba</PRE>
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<H4>Character sets:</H4>
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<P>A character set is a bracket-expression starting with [ and ending with ], it
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defines a set of characters, and matches any single character that is a member
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of that set.</P>
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<P>A bracket expression may contain any combination of the following:</P>
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<BLOCKQUOTE dir="ltr" style="MARGIN-RIGHT: 0px">
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<H5>Single characters:</H5>
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<P>For example [abc], will match any of the characters 'a', 'b', or 'c'.</P>
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<H5>Character ranges:</H5>
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<P>For example [a-c] will match any single character in the range 'a' to
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'c'. By default, for POSIX-Basic regular expressions, a character <EM>x</EM>
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is within the range <EM>y</EM> to <EM>z</EM>, if it collates within that
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range; this results in locale specific behavior. This behavior can
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be turned off by unsetting the <EM><A href="syntax_option_type.html#basic">collate</A></EM>
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option flag - in which case whether a character appears within a range is
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determined by comparing the code points of the characters only</P>
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<H5>Negation:</H5>
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<P>If the bracket-expression begins with the ^ character, then it matches the
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complement of the characters it contains, for example [^a-c] matches any
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character that is not in the range a-c.</P>
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<H5>Character classes:</H5>
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<P>An expression of the form [[:name:]] matches the named character class "name",
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for example [[:lower:]] matches any lower case character. See <A href="character_class_names.html">
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character class names</A>.</P>
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<H5>Collating Elements:</H5>
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<P>An expression of the form [[.col.] matches the collating element <EM>col</EM>.
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A collating element is any single character, or any sequence of characters that
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collates as a single unit. Collating elements may also be used as the end
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point of a range, for example: [[.ae.]-c] matches the character sequence "ae",
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plus any single character in the rangle "ae"-c, assuming that "ae" is treated
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as a single collating element in the current locale.</P>
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2005-04-16 16:06:45 +00:00
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<P>Collating elements may be used in place of escapes (which are not normally
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allowed inside character sets), for example [[.^.]abc] would match either one
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of the characters 'abc^'.</P>
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<P>As an extension, a collating element may also be specified via its <A href="collating_names.html">
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symbolic name</A>, for example:</P>
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<P>[[.NUL.]]</P>
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<P>matches a NUL character.</P>
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<H5>Equivalence classes:</H5>
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<P>
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An expression of theform[[=col=]], matches any character or collating element
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whose primary sort key is the same as that for collating element <EM>col</EM>,
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as with collating elements the name <EM>col</EM> may be a <A href="collating_names.html">
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symbolic name</A>. A primary sort key is one that ignores case,
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accentation, or locale-specific tailorings; so for example [[=a=]] matches any
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of the characters: a, <20>, <20>, <20>, <20>, <20>, <20>, A, <20>, <20>, <20>, <20>, <20> and <20>.
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Unfortunately implementation of this is reliant on the platform's collation and
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localisation support; this feature can not be relied upon to work portably
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across all platforms, or even all locales on one platform.</P>
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</BLOCKQUOTE>
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<H5>Combinations:</H5>
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<P>All of the above can be combined in one character set declaration, for example:
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[[:digit:]a-c[.NUL.]].</P>
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<H4>Escapes</H4>
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<P>With the exception of the escape sequences \{, \}, \(, and \), which are
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documented above, an escape followed by any character matches that
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character. This can be used to make the special characters .[\*^$,
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"ordinary". Note that the escape character loses its special meaning
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inside a character set, so [\^] will match either a literal '\' or a '^'.</P>
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<H4>What Gets Matched</H4>
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<P>When there is more that one way to match a regular expression, the "best"
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possible match is obtained using the <A href="syntax_leftmost_longest.html">leftmost-longest
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rule</A>.</P>
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<H3><A name="variations"></A>Variations</H3>
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<H4><A name="grep"></A>Grep</H4>
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<P>When an expression is compiled with the flag <EM>grep</EM> set, then the
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expression is treated as a newline separated list of <A href="#Basic">POSIX-Basic</A>
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expressions, a match is found if any of the expressions in the list match, for
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example:</P>
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<PRE>boost::regex e("abc\ndef", boost::regex::grep);</PRE>
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<P>will match either of the POSIX-Basic expressions "abc" or "def".</P>
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<P>As its name suggests, this behavior is consistent with the Unix utility <EM>grep</EM>.</P>
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<H4><A name="emacs"></A>emacs</H4>
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<P>In addition to the <A href="#Basic">POSIX-Basic features</A> the following
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characters are also special:</P>
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<BLOCKQUOTE dir="ltr" style="MARGIN-RIGHT: 0px">
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<P>+ repeats the preceding atom one or more times.</P>
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<P>? repeats the preceding atom zero or one times.</P>
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<P>*? A non-greedy version of *.</P>
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<P>+? A non-greedy version of +.</P>
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<P>?? A non-greedy version of ?.</P>
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</BLOCKQUOTE>
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<P>And the following escape sequences are also recognised:</P>
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<BLOCKQUOTE dir="ltr" style="MARGIN-RIGHT: 0px">
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<P>\| specifies an alternative.</P>
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<P>\(?: ... \) is a non-marking grouping construct - allows you to
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lexically group something without spitting out an extra sub-expression.</P>
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<P>\w matches any word character.</P>
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<P>\W matches any non-word character.</P>
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<P>\sx matches any character in the syntax group <EM>x</EM>, the following emacs
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groupings are supported: 's', ' ', '_', 'w', '.', ')', '(', '"', '\'', '>'
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and '<'. Refer to the emacs docs for details.</P>
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<P>\Sx matches any character not in the syntax grouping <EM>x</EM>.</P>
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<P>\c and \C are not supported.</P>
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<P>\` matches zero characters only at the start of a buffer (or string being
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matched).</P>
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<P>\' matches zero characters only at the end of a buffer (or string being
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matched).</P>
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<P>\b matches zero characters at a word boundary.</P>
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<P>\B matches zero characters, not at a word boundary.</P>
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<P>\< matches zero characters only at the start of a word.</P>
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<P>\> matches zero characters only at the end of a word.</P>
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</BLOCKQUOTE>
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<P dir="ltr">Finally, you should note that emacs style regular expressions are
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matched according to the <A href="syntax_perl.html#what">Perl "depth first search"
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rules</A>. Emacs expressions are matched this way because they contain
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Perl-like extensions, that do not interact well with the <A href="syntax_leftmost_longest.html">
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POSIX-style leftmost-longest rule</A>.</P>
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<H3><A name="options"></A>Options</H3>
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<P>There are a <A href="syntax_option_type.html#basic">variety of flags</A> that
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may be combined with the <EM>basic</EM> and <EM>grep</EM> options when
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constructing the regular expression, in particular note that the <A href="syntax_option_type.html#basic">
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newline_alt, no_char_classes, no-intervals, bk_plus_qm and bk_plus_vbar</A> options
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all alter the syntax, while the <A href="syntax_option_type.html#basic">collate
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and icase</A> options modify how the case and locale sensitivity are to be
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applied.</P>
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<H3><A name="refs"></A>References</H3>
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<P><A href="http://www.opengroup.org/onlinepubs/000095399/basedefs/xbd_chap09.html">IEEE
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Std 1003.1-2001, Portable Operating System Interface (POSIX ), Base Definitions
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and Headers, Section 9, Regular Expressions (FWD.1).</A></P>
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<P><A href="http://www.opengroup.org/onlinepubs/000095399/utilities/grep.html">IEEE
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Std 1003.1-2001, Portable Operating System Interface (POSIX ), Shells and
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Utilities, Section 4, Utilities, grep (FWD.1).</A></P>
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<P><A href="http://www.gnu.org/software/emacs/">Emacs Version 21.3</A>.</P>
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<P>
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<HR>
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<P></P>
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<p>Revised
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<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
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21 Aug 2004
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<!--webbot bot="Timestamp" endspan i-checksum="39359" --></p>
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<P><I><EFBFBD> Copyright <a href="mailto:jm@regex.fsnet.co.uk">John Maddock</a> 2004</I></P>
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<I>
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<P><I>Use, modification and distribution are subject to the Boost Software License,
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Version 1.0. (See accompanying file <A href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</A>
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or copy at <A href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>).</I></P>
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</I>
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