2003-05-17 11:45:48 +00:00
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<title>Boost.Regex: Format String Syntax</title>
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<h3><a href="../../../index.htm"><img height="86" width="277" alt=
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<td width="353">
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<h1 align="center">Boost.Regex</h1>
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<h2 align="center">Format String 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" width="43" alt=
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"Boost.Regex Index" src="uarrow.gif" border="0"></a></h3>
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<br>
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<hr>
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2003-05-24 11:13:26 +00:00
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<p>Format strings are used by the algorithm <a href="regex_replace.html">regex_replace</a> and by <a
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2003-05-17 11:45:48 +00:00
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href="match_results.html">match_results::format</a>, and are used
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to transform one string into another.</p>
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<p>There are three kind of format string: sed, Perl and extended,
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the extended syntax is a superset of the others so this is covered
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first.</p>
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<p><b><i>Extended format syntax</i></b></p>
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<p>In format strings, all characters are treated as literals
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except: ()$\?:</p>
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<p>To use any of these as literals you must prefix them with the
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escape character \</p>
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<p>The following special sequences are recognized: <br>
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<br>
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<i>Grouping:</i></p>
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<p>Use the parenthesis characters ( and ) to group sub-expressions
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within the format string, use \( and \) to represent literal '('
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and ')'. <br>
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<br>
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<i>Sub-expression expansions:</i></p>
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<p>The following Perl like expressions expand to a particular
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matched sub-expression:<br>
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</p>
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<p></p>
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<table id="Table2" cellspacing="0" cellpadding="7" width="100%"
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border="0">
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<tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">$`</td>
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<td valign="top" width="43%">Expands to all the text from the end
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of the previous match to the start of the current match, if there
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was no previous match in the current operation, then everything
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from the start of the input string to the start of the match.</td>
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<td valign="top" width="9%"> </td>
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</tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">$'</td>
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<td valign="top" width="43%">Expands to all the text from the end
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of the match to the end of the input string.</td>
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<td valign="top" width="9%"> </td>
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</tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">$&</td>
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<td valign="top" width="43%">Expands to all of the current
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match.</td>
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<td valign="top" width="9%"> </td>
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</tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">$0</td>
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<td valign="top" width="43%">Expands to all of the current
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match.</td>
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<td valign="top" width="9%"> </td>
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</tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">$N</td>
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<td valign="top" width="43%">Expands to the text that matched
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sub-expression <i>N</i>.</td>
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<td valign="top" width="9%"> </td>
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</table>
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<br>
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<p><i>Conditional expressions:</i></p>
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<p>Conditional expressions allow two different format strings to be
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selected dependent upon whether a sub-expression participated in
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the match or not:</p>
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<p>?Ntrue_expression:false_expression</p>
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<p>Executes true_expression if sub-expression <i>N</i> participated
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in the match, otherwise executes false_expression.</p>
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<p>Example: suppose we search for "(while)|(for)" then the format
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string "?1WHILE:FOR" would output what matched, but in upper
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case. <br>
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<br>
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<i>Escape sequences:</i></p>
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<p>The following escape sequences are also allowed:<br>
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</p>
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<p></p>
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<table id="Table3" cellspacing="0" cellpadding="7" width="100%"
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border="0">
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<tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">\a</td>
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<td valign="top" width="43%">The bell character.</td>
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<td valign="top" width="9%"> </td>
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</tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">\f</td>
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<td valign="top" width="43%">The form feed character.</td>
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<td valign="top" width="9%"> </td>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">\n</td>
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<td valign="top" width="43%">The newline character.</td>
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<td valign="top" width="9%"> </td>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">\r</td>
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<td valign="top" width="43%">The carriage return character.</td>
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<td valign="top" width="9%"> </td>
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</tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">\t</td>
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<td valign="top" width="43%">The tab character.</td>
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<td valign="top" width="9%"> </td>
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</tr>
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<tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">\v</td>
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<td valign="top" width="43%">A vertical tab character.</td>
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<td valign="top" width="9%"> </td>
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</tr>
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<tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">\x</td>
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<td valign="top" width="43%">A hexadecimal character - for example
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\x0D.</td>
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<td valign="top" width="9%"> </td>
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</tr>
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<tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">\x{}</td>
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<td valign="top" width="43%">A possible Unicode hexadecimal
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character - for example \x{1A0}</td>
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<td valign="top" width="9%"> </td>
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</tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">\cx</td>
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<td valign="top" width="43%">The ASCII escape character x, for
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example \c@ is equivalent to escape-@.</td>
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<td valign="top" width="9%"> </td>
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</tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">\e</td>
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<td valign="top" width="43%">The ASCII escape character.</td>
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<td valign="top" width="9%"> </td>
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</tr>
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<td valign="top" width="8%"> </td>
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<td valign="top" width="40%">\dd</td>
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<td valign="top" width="43%">An octal character constant, for
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example \10.</td>
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<td valign="top" width="9%"> </td>
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</tr>
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</table>
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<br>
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<br>
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<p><b><i>Perl format strings</i></b></p>
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<p>Perl format strings are the same as the default syntax except
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that the characters ()?: have no special meaning.</p>
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<p><b><i>Sed format strings</i></b></p>
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<p>Sed format strings use only the characters \ and & as
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special characters.</p>
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<p>\n where n is a digit, is expanded to the nth
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sub-expression.</p>
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<p>& is expanded to the whole of the match (equivalent to
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\0).</p>
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<p>Other escape sequences are expanded as per the default
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syntax.</p>
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<p></p>
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<hr>
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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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17 May 2003
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<!--webbot bot="Timestamp" endspan i-checksum="39359" --></p>
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<p><i>© Copyright <a href="mailto:jm@regex.fsnet.co.uk">John
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Maddock</a> 1998-
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<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%Y" startspan -->
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2003
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<!--webbot bot="Timestamp" endspan i-checksum="39359" --></i></p>
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<p align="left"><i>Permission to use, copy, modify, distribute and
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sell this software and its documentation for any purpose is hereby
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granted without fee, provided that the above copyright notice
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appear in all copies and that both that copyright notice and this
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permission notice appear in supporting documentation. Dr John
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Maddock makes no representations about the suitability of this
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software for any purpose. It is provided "as is" without express or
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implied warranty.</i></p>
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</body>
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</html>
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