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John Maddock b912fb5b64 Improve partial match docs. 
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>Boost.Regex: Partial Matches</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" type="text/css" href="../../../boost.css">
</head>
<body>
<P>
<TABLE id="Table1" cellSpacing="1" cellPadding="1" width="100%" border="0">
<TR>
<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>
</td>
<TD width="353">
<H1 align="center">Boost.Regex</H1>
<H2 align="center">Partial Matches</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>
</td>
</TR>
</TABLE>
</P>
<HR>
<p></p>
<P>The <A href="match_flag_type.html">match-flag</A> <CODE>match_partial</CODE> can
be passed to the following algorithms: <A href="regex_match.html">regex_match</A>,
<A href="regex_search.html">regex_search</A>, and <A href="regex_grep.html">regex_grep</A>,
and used with the iterator <A href="regex_iterator.html">regex_iterator</A>.
When used it indicates that partial as well as full matches should be found. A
partial match is one that matched one or more characters at the end of the text
input, but did not match all of the regular expression (although it may have
done so had more input been available). Partial matches are typically used when
either validating data input (checking each character as it is entered on the
keyboard), or when searching texts that are either too long to load into memory
(or even into a memory mapped file), or are of indeterminate length (for
example the source may be a socket or similar). Partial and full matches can be
differentiated as shown in the following table (the variable M represents an
instance of <A href="match_results.html">match_results&lt;&gt;</A> as filled in
by regex_match, regex_search or regex_grep):<BR>
</P>
<P>
<TABLE id="Table2" cellSpacing="0" cellPadding="7" width="100%" border="0">
<TR>
<TD vAlign="top" width="20%">&nbsp;</TD>
<TD vAlign="top" width="20%">Result</TD>
<TD vAlign="top" width="20%">M[0].matched</TD>
<TD vAlign="top" width="20%">M[0].first</TD>
<TD vAlign="top" width="20%">M[0].second</TD>
</TR>
<TR>
<TD vAlign="top" width="20%">No match</TD>
<TD vAlign="top" width="20%">False</TD>
<TD vAlign="top" width="20%">Undefined</TD>
<TD vAlign="top" width="20%">Undefined</TD>
<TD vAlign="top" width="20%">Undefined</TD>
</TR>
<TR>
<TD vAlign="top" width="20%">Partial match</TD>
<TD vAlign="top" width="20%">True</TD>
<TD vAlign="top" width="20%">False</TD>
<TD vAlign="top" width="20%">Start of partial match.</TD>
<TD vAlign="top" width="20%">End of partial match (end of text).</TD>
</TR>
<TR>
<TD vAlign="top" width="20%">Full match</TD>
<TD vAlign="top" width="20%">True</TD>
<TD vAlign="top" width="20%">True</TD>
<TD vAlign="top" width="20%">Start of full match.</TD>
<TD vAlign="top" width="20%">End of full match.</TD>
</TR>
</TABLE>
</P>
<P>Be aware that using partial matches can sometimes result in somewhat imperfect
behavior:</P>
<UL>
<LI>
There are some expressions, such as ".*abc" that will always produce a partial
match.&nbsp; This problem can be reduced by careful construction of the regular
expressions used, or by setting flags like match_not_dot_newline so that
expressions like .* can't match past line boundaries.</LI>
<LI>
Boost.Regex currently prefers leftmost matches to full matches, so for example
matching "abc|b" against "ab" produces a partial match&nbsp;against the "ab"
rather than a full match against "b".&nbsp; It's more efficient to work this
way, but may not be the behavior you want in all situations.</LI></UL>
<P>The following <A href="../example/snippets/partial_regex_match.cpp">example</A>
tests to see whether the text could be a valid credit card number, as the user
presses a key, the character entered would be added to the string being built
up, and passed to <CODE>is_possible_card_number</CODE>. If this returns true
then the text could be a valid card number, so the user interface's OK button
would be enabled. If it returns false, then this is not yet a valid card
number, but could be with more input, so the user interface would disable the
OK button. Finally, if the procedure throws an exception the input could never
become a valid number, and the inputted character must be discarded, and a
suitable error indication displayed to the user.</P>
<PRE>#include &lt;string&gt;
#include &lt;iostream&gt;
#include &lt;boost/regex.hpp&gt;
boost::regex e("(\\d{3,4})[- ]?(\\d{4})[- ]?(\\d{4})[- ]?(\\d{4})");
bool is_possible_card_number(const std::string&amp; input)
{
//
// return false for partial match, true for full match, or throw for
// impossible match based on what we have so far...
boost::match_results&lt;std::string::const_iterator&gt; what;
if(0 == boost::regex_match(input, what, e, boost::match_default | boost::match_partial))
{
// the input so far could not possibly be valid so reject it:
throw std::runtime_error("Invalid data entered - this could not possibly be a valid card number");
}
// OK so far so good, but have we finished?
if(what[0].matched)
{
// excellent, we have a result:
return true;
}
// what we have so far is only a partial match...
return false;
}</PRE>
<P>In the following <A href="../example/snippets/partial_regex_grep.cpp">example</A>,
text input is taken from a stream containing an unknown amount of text; this
example simply counts the number of html tags encountered in the stream. The
text is loaded into a buffer and searched a part at a time, if a partial match
was encountered, then the partial match gets searched a second time as the
start of the next batch of text:</P>
<PRE>#include &lt;iostream&gt;
#include &lt;fstream&gt;
#include &lt;sstream&gt;
#include &lt;string&gt;
#include &lt;boost/regex.hpp&gt;
// match some kind of html tag:
boost::regex e("&lt;[^&gt;]*&gt;");
// count how many:
unsigned int tags = 0;
// saved position of partial match:
char* next_pos = 0;
bool grep_callback(const boost::match_results&lt;char*&gt;&amp; m)
{
if(m[0].matched == false)
{
// save position and return:
next_pos = m[0].first;
}
else
++tags;
return true;
}
void search(std::istream&amp; is)
{
char buf[4096];
next_pos = buf + sizeof(buf);
bool have_more = true;
while(have_more)
{
// how much do we copy forward from last try:
unsigned leftover = (buf + sizeof(buf)) - next_pos;
// and how much is left to fill:
unsigned size = next_pos - buf;
// copy forward whatever we have left:
memcpy(buf, next_pos, leftover);
// fill the rest from the stream:
unsigned read = is.readsome(buf + leftover, size);
// check to see if we've run out of text:
have_more = read == size;
// reset next_pos:
next_pos = buf + sizeof(buf);
// and then grep:
boost::regex_grep(grep_callback,
buf,
buf + read + leftover,
e,
boost::match_default | boost::match_partial);
}
}</PRE>
<P>
<HR>
<P></P>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
24 Oct 2003
<!--webbot bot="Timestamp" endspan i-checksum="39359" --></p>
<p><i><EFBFBD> Copyright John Maddock&nbsp;1998-
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%Y" startspan --> 2003<!--webbot bot="Timestamp" endspan i-checksum="39359" --></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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