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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<title>Boost.Regex: Boost-Extended Format String Syntax</title>
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<P>
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<td valign="top" width="300">
<h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3>
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<TD width="353">
<H1 align="center">Boost.Regex</H1>
<H2 align="center">Boost-Extended Format String Syntax</H2>
</TD>
<td width="50">
<h3><a href="index.html"><img height="45" width="43" alt="Boost.Regex Index" src="uarrow.gif" border="0"></a></h3>
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</TABLE>
</P>
<P>Boost-Extended format strings treat all characters as literals except for
'$', '\', '(', ')', '?', ':' and '\'.</P>
<H4>Grouping</H4>
<P>The characters '(' and ')' perform lexical grouping, use \( and \) if you want
a to output literal parenthesis.</P>
<H4>Conditionals</H4>
<P>The character '?' begins a conditional expression, the general form is:</P>
<PRE>?Ntrue-expression:false-expression</PRE>
<P>where N is decimal digit.</P>
<P>If sub-expression <EM>N</EM> was matched, then true-expression is evaluated and
sent to output, otherwise false-expression is evaluated and sent to output.</P>
<P>You will normally need to surround a conditional-expression with parenthesis in
order to prevent ambiguities.</P>
<H4>Placeholder Sequences</H4>
<P>Placeholder sequences specify that some part of what matched the regular
expression should be sent to output as follows:</P>
<P>
<TABLE id="Table2" cellSpacing="4" cellPadding="1" width="100%" border="0">
<TR>
<TD><STRONG>Placeholder</STRONG></TD>
<TD><STRONG>Meaning</STRONG></TD>
</TR>
<TR>
<TD>$&</TD>
<TD>Outputs what matched the whole expression.</TD>
</TR>
<TR>
<TD>$`</TD>
<TD>Outputs the text between the end of the last match found (or the start of the
text if no previous match was found), and the start of the current match.</TD>
</TR>
<TR>
<TD>$'</TD>
<TD>Outputs all the text following the end of the current match.</TD>
</TR>
<TR>
<TD>$$</TD>
<TD>Outputs a literal '$'</TD>
</TR>
<TR>
<TD>$n</TD>
<TD>Outputs what matched the n'th sub-expression.</TD>
</TR>
</TABLE>
</P>
<P>Any $-placeholder sequence not listed above, results in '$' being treated as a
literal.</P>
<H4>Escape Sequences</H4>
<P>An escape character followed by any character <EM>x</EM>, outputs that
character unless <EM>x</EM> is one of the escape sequences shown below.</P>
<P>
<TABLE id="Table3" cellSpacing="4" cellPadding="1" width="100%" border="0">
<TR>
<TD><STRONG>Escape</STRONG></TD>
<TD><STRONG>Meaning</STRONG></TD>
</TR>
<TR>
<TD>\a</TD>
<TD>Outputs the bell character: '\a'.</TD>
</TR>
<TR>
<TD>\e</TD>
<TD>Outputs the ANSI escape character (code point 27).</TD>
</TR>
<TR>
<TD>\f</TD>
<TD>Outputs a form feed character: '\f'</TD>
</TR>
<TR>
<TD>\n</TD>
<TD>Outputs a newline character: '\n'.</TD>
</TR>
<TR>
<TD>\r</TD>
<TD>Outputs a carriage return character: '\r'.</TD>
</TR>
<TR>
<TD>\t</TD>
<TD>Outputs a tab character: '\t'.</TD>
</TR>
<TR>
<TD>\v</TD>
<TD>Outputs a vertical tab character: '\v'.</TD>
</TR>
<TR>
<TD>\xDD</TD>
<TD>Outputs the character whose hexadecimal code point is 0xDD</TD>
</TR>
<TR>
<TD>\x{DDDD}</TD>
<TD>Outputs the character whose hexadecimal code point is 0xDDDDD</TD>
</TR>
<TR>
<TD>\cX</TD>
<TD>Outputs the ANSI escape sequence "escape-X".</TD>
</TR>
<TR>
<TD>\D</TD>
<TD>If <EM>D</EM> is a decimal digit in the range 1-9, then outputs the text that
matched sub-expression <EM>D</EM>.</TD>
</TR>
<TR>
<TD>\l</TD>
<TD>Causes the next character to be outputted, to be output in lower case.</TD>
</TR>
<TR>
<TD>\u</TD>
<TD>Causes the next character to be outputted, to be output in upper case.</TD>
</TR>
<TR>
<TD>\L</TD>
<TD>Causes all subsequent characters to be output in lower case, until a \E is
found.</TD>
</TR>
<TR>
<TD>\U</TD>
<TD>Causes all subsequent characters to be output in upper case, until a \E is
found.</TD>
</TR>
<TR>
<TD>\E</TD>
<TD>Terminates a \L or \U sequence.</TD>
</TR>
</TABLE>
</P>
<P>
<HR>
<P></P>
<P></P>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
24 Nov 2004
<!--webbot bot="Timestamp" endspan i-checksum="39359" --></p>
<p><i> Copyright John Maddock 2004</i></p>
<P><I>Use, modification and distribution are subject to the Boost Software License,
Version 1.0. (See accompanying file <A href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</A>
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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