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<title>Issue 2359: How does regex_constants::nosubs affect basic_regex::mark_count()?</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#C++14">C++14</a> status.</em></p>
<h3 id="2359"><a href="lwg-defects.html#2359">2359</a>. How does <code>regex_constants::nosubs</code> affect <code>basic_regex::mark_count()</code>?</h3>
<p><b>Section:</b> 28.6.4.2 <a href="https://wg21.link/re.synopt">[re.synopt]</a> <b>Status:</b> <a href="lwg-active.html#C++14">C++14</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2014-02-01 <b>Last modified:</b> 2017-09-07</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#re.synopt">issues</a> in [re.synopt].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++14">C++14</a> status.</p>
<p><b>Discussion:</b></p>
<p>
As discussed in <a href="http://listarchives.isocpp.org/cgi-bin/wg21/message?wg=lib&amp;msg=35399">c++std-lib-35399</a> and its replies, 
I can see two possible interpretations of the effects of <code>regex_constants::nosubs</code>:
</p>
<ol style="list-style-type:decimal">
<li><p>
The effect of <code>nosubs</code> only applies during matching. Parentheses are
still recognized as marking a sub-expression by the <code>basic_regex</code>
compiler, and <code>basic_regex::mark_count()</code> still returns the number of
marked sub-expressions, but anything they match is not stored in the
results. This means it is not always true that <code>results.size() == r.mark_count() + 1</code> 
for a successful match.
</p></li>
<li><p>
<code>nosubs</code> affects how a regular expression is compiled, altering the
state of the <code>std::basic_regex</code> object so that <code>mark_count()</code> returns
zero. This also affects any subsequent matching.
</p></li>
</ol>
<p>
The definition of <code>nosubs</code> should make this clear.
<p/>
The wording in 28.6.4.2 <a href="https://wg21.link/re.synopt">[re.synopt]</a>/1 seems to imply that <code>nosubs</code> only has
effects during matching, which is (1), but all known implementations
do (2). John Maddock confirmed that (2) was intended.
</p>

<p><i>[Issaquah 2014-02-12: Move to Immediate]</i></p>




<p id="res-2359"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3797.</p>

<ol>
<li><p>Apply the following edit to the table in 28.6.4.2 <a href="https://wg21.link/re.synopt">[re.synopt]</a>/1</p>

<blockquote><p>
Specifies that <ins>no sub-expressions shall be considered to be
marked, so that</ins> when a regular expression is matched against a character container
sequence, no sub-expression matches shall be stored in the supplied
<code>match_results</code> structure.
</p></blockquote>
</li>
</ol>






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