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| <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> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 1998 -2010 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> |
| </div></td> |
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