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<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_regex.background_information.faq"></a><a class="link" href="faq.html" title="FAQ">FAQ</a>
</h3></div></div></div>
<p>
<span class="bold"><strong>Q.</strong></span> I can't get regex++ to work with escape
characters, what's going on?
</p>
<p>
<span class="bold"><strong>A.</strong></span> If you embed regular expressions in C++
code, then remember that escape characters are processed twice: once by the
C++ compiler, and once by the Boost.Regex expression compiler, so to pass
the regular expression \d+ to Boost.Regex, you need to embed "\d+"
in your code. Likewise to match a literal backslash you will need to embed
"\\" in your code.
</p>
<p>
<span class="bold"><strong>Q.</strong></span> No matter what I do regex_match always
returns false, what's going on?
</p>
<p>
<span class="bold"><strong>A.</strong></span> The algorithm regex_match only succeeds
if the expression matches <span class="bold"><strong>all</strong></span> of the text,
if you want to <span class="bold"><strong>find</strong></span> a sub-string within
the text that matches the expression then use regex_search instead.
</p>
<p>
<span class="bold"><strong>Q.</strong></span> Why does using parenthesis in a POSIX
regular expression change the result of a match?
</p>
<p>
<span class="bold"><strong>A.</strong></span> For POSIX (extended and basic) regular
expressions, but not for perl regexes, parentheses don't only mark; they
determine what the best match is as well. When the expression is compiled
as a POSIX basic or extended regex then Boost.Regex follows the POSIX standard
leftmost longest rule for determining what matched. So if there is more than
one possible match after considering the whole expression, it looks next
at the first sub-expression and then the second sub-expression and so on.
So...
</p>
<p>
"(0*)([0-9]*)" against "00123" would produce $1 = "00"
$2 = "123"
</p>
<p>
where as
</p>
<p>
"0*([0-9])*" against "00123" would produce $1 = "00123"
</p>
<p>
If you think about it, had $1 only matched the "123", this would
be "less good" than the match "00123" which is both further
to the left and longer. If you want $1 to match only the "123"
part, then you need to use something like:
</p>
<p>
"0*([1-9][0-9]*)"
</p>
<p>
as the expression.
</p>
<p>
<span class="bold"><strong>Q.</strong></span> Why don't character ranges work properly
(POSIX mode only)?
</p>
<p>
<span class="bold"><strong>A.</strong></span> The POSIX standard specifies that character
range expressions are locale sensitive - so for example the expression [A-Z]
will match any collating element that collates between 'A' and 'Z'. That
means that for most locales other than "C" or "POSIX",
[A-Z] would match the single character 't' for example, which is not what
most people expect - or at least not what most people have come to expect
from regular expression engines. For this reason, the default behaviour of
Boost.Regex (perl mode) is to turn locale sensitive collation off by not
setting the <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">collate</span></code>
compile time flag. However if you set a non-default compile time flag - for
example <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">extended</span></code> or <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">basic</span></code>,
then locale dependent collation will be enabled, this also applies to the
POSIX API functions which use either <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">extended</span></code>
or <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">basic</span></code> internally. [Note - when <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">nocollate</span></code> in effect, the library behaves
"as if" the LC_COLLATE locale category were always "C",
regardless of what its actually set to - end note].
</p>
<p>
<span class="bold"><strong>Q.</strong></span> Why are there no throw specifications
on any of the functions? What exceptions can the library throw?
</p>
<p>
<span class="bold"><strong>A.</strong></span> Not all compilers support (or honor)
throw specifications, others support them but with reduced efficiency. Throw
specifications may be added at a later date as compilers begin to handle
this better. The library should throw only three types of exception: [boost::regex_error]
can be thrown by <a class="link" href="../ref/basic_regex.html" title="basic_regex"><code class="computeroutput"><span class="identifier">basic_regex</span></code></a> when compiling a regular
expression, <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">runtime_error</span></code> can be thrown when a call
to <code class="computeroutput"><span class="identifier">basic_regex</span><span class="special">::</span><span class="identifier">imbue</span></code> tries to open a message catalogue
that doesn't exist, or when a call to <a class="link" href="../ref/regex_search.html" title="regex_search"><code class="computeroutput"><span class="identifier">regex_search</span></code></a> or <a class="link" href="../ref/regex_match.html" title="regex_match"><code class="computeroutput"><span class="identifier">regex_match</span></code></a> results in an "everlasting"
search, or when a call to <code class="computeroutput"><span class="identifier">RegEx</span><span class="special">::</span><span class="identifier">GrepFiles</span></code>
or <code class="computeroutput"><span class="identifier">RegEx</span><span class="special">::</span><span class="identifier">FindFiles</span></code> tries to open a file that cannot
be opened, finally <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">bad_alloc</span></code> can be thrown by just about any
of the functions in this library.
</p>
<p>
<span class="bold"><strong>Q.</strong></span> Why can't I use the "convenience"
versions of regex_match / regex_search / regex_grep / regex_format / regex_merge?
</p>
<p>
<span class="bold"><strong>A.</strong></span> These versions may or may not be available
depending upon the capabilities of your compiler, the rules determining the
format of these functions are quite complex - and only the versions visible
to a standard compliant compiler are given in the help. To find out what
your compiler supports, run <boost/regex.hpp> through your C++ pre-processor,
and search the output file for the function that you are interested in. Note
however, that very few current compilers still have problems with these overloaded
functions.
</p>
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<td align="right"><div class="copyright-footer">Copyright © 1998-2013 John Maddock<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
</p>
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