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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_regex.background.faq"></a><a class="link" href="faq.html" title="FAQ">FAQ</a>
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</h3></div></div></div>
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<p>
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<span class="bold"><strong>Q.</strong></span> I can't get regex++ to work with escape
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characters, what's going on?
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</p>
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<p>
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<span class="bold"><strong>A.</strong></span> If you embed regular expressions in C++
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code, then remember that escape characters are processed twice: once by the
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C++ compiler, and once by the Boost.Regex expression compiler, so to pass
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the regular expression \d+ to Boost.Regex, you need to embed "\d+"
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in your code. Likewise to match a literal backslash you will need to embed
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"\\" in your code.
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</p>
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<p>
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<span class="bold"><strong>Q.</strong></span> No matter what I do regex_match always
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returns false, what's going on?
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</p>
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<p>
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<span class="bold"><strong>A.</strong></span> The algorithm regex_match only succeeds
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if the expression matches <span class="bold"><strong>all</strong></span> of the text,
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if you want to <span class="bold"><strong>find</strong></span> a sub-string within
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the text that matches the expression then use regex_search instead.
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</p>
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<p>
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<span class="bold"><strong>Q.</strong></span> Why does using parenthesis in a POSIX
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regular expression change the result of a match?
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</p>
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<p>
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<span class="bold"><strong>A.</strong></span> For POSIX (extended and basic) regular
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expressions, but not for perl regexes, parentheses don't only mark; they
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determine what the best match is as well. When the expression is compiled
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as a POSIX basic or extended regex then Boost.Regex follows the POSIX standard
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leftmost longest rule for determining what matched. So if there is more than
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one possible match after considering the whole expression, it looks next
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at the first sub-expression and then the second sub-expression and so on.
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So...
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</p>
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<p>
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"(0*)([0-9]*)" against "00123" would produce $1 = "00"
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$2 = "123"
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</p>
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<p>
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where as
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</p>
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<p>
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"0*([0-9])*" against "00123" would produce $1 = "00123"
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</p>
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<p>
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If you think about it, had $1 only matched the "123", this would
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be "less good" than the match "00123" which is both further
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to the left and longer. If you want $1 to match only the "123"
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part, then you need to use something like:
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</p>
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<p>
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"0*([1-9][0-9]*)"
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</p>
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<p>
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as the expression.
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</p>
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<p>
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<span class="bold"><strong>Q.</strong></span> Why don't character ranges work properly
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(POSIX mode only)?
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</p>
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<p>
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<span class="bold"><strong>A.</strong></span> The POSIX standard specifies that character
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range expressions are locale sensitive - so for example the expression [A-Z]
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will match any collating element that collates between 'A' and 'Z'. That
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means that for most locales other than "C" or "POSIX",
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[A-Z] would match the single character 't' for example, which is not what
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most people expect - or at least not what most people have come to expect
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from regular expression engines. For this reason, the default behaviour of
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Boost.Regex (perl mode) is to turn locale sensitive collation off by not
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setting the <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">collate</span></code>
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compile time flag. However if you set a non-default compile time flag - for
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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>,
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then locale dependent collation will be enabled, this also applies to the
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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>
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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
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"as if" the LC_COLLATE locale category were always "C",
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regardless of what its actually set to - end note].
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</p>
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<p>
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<span class="bold"><strong>Q.</strong></span> Why are there no throw specifications
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on any of the functions? What exceptions can the library throw?
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</p>
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<p>
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<span class="bold"><strong>A.</strong></span> Not all compilers support (or honor)
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throw specifications, others support them but with reduced efficiency. Throw
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specifications may be added at a later date as compilers begin to handle
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this better. The library should throw only three types of exception: [boost::regex_error]
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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
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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
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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
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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"
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search, or when a call to <code class="computeroutput"><span class="identifier">RegEx</span><span class="special">::</span><span class="identifier">GrepFiles</span></code>
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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
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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
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of the functions in this library.
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</p>
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<p>
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<span class="bold"><strong>Q.</strong></span> Why can't I use the "convenience"
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versions of regex_match / regex_search / regex_grep / regex_format / regex_merge?
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</p>
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<p>
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<span class="bold"><strong>A.</strong></span> These versions may or may not be available
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depending upon the capabilities of your compiler, the rules determining the
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format of these functions are quite complex - and only the versions visible
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to a standard compliant compiler are given in the help. To find out what
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your compiler supports, run <boost/regex.hpp> through your C++ pre-processor,
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and search the output file for the function that you are interested in. Note
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however, that very few current compilers still have problems with these overloaded
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functions.
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 1998-2013 John Maddock<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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</p>
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</div></td>
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