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| 24 | </div> | |
| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h3 class="title"> | |
| 27 | <a name="boost_regex.background_information.faq"></a><a class="link" href="faq.html" title="FAQ">FAQ</a> | |
| 28 | </h3></div></div></div> | |
| 29 | <p> | |
| 30 | <span class="bold"><strong>Q.</strong></span> I can't get regex++ to work with escape | |
| 31 | characters, what's going on? | |
| 32 | </p> | |
| 33 | <p> | |
| 34 | <span class="bold"><strong>A.</strong></span> If you embed regular expressions in C++ | |
| 35 | code, then remember that escape characters are processed twice: once by the | |
| 36 | C++ compiler, and once by the Boost.Regex expression compiler, so to pass | |
| 37 | the regular expression \d+ to Boost.Regex, you need to embed "\d+" | |
| 38 | in your code. Likewise to match a literal backslash you will need to embed | |
| 39 | "\\" in your code. | |
| 40 | </p> | |
| 41 | <p> | |
| 42 | <span class="bold"><strong>Q.</strong></span> No matter what I do regex_match always | |
| 43 | returns false, what's going on? | |
| 44 | </p> | |
| 45 | <p> | |
| 46 | <span class="bold"><strong>A.</strong></span> The algorithm regex_match only succeeds | |
| 47 | if the expression matches <span class="bold"><strong>all</strong></span> of the text, | |
| 48 | if you want to <span class="bold"><strong>find</strong></span> a sub-string within | |
| 49 | the text that matches the expression then use regex_search instead. | |
| 50 | </p> | |
| 51 | <p> | |
| 52 | <span class="bold"><strong>Q.</strong></span> Why does using parenthesis in a POSIX | |
| 53 | regular expression change the result of a match? | |
| 54 | </p> | |
| 55 | <p> | |
| 56 | <span class="bold"><strong>A.</strong></span> For POSIX (extended and basic) regular | |
| 57 | expressions, but not for perl regexes, parentheses don't only mark; they | |
| 58 | determine what the best match is as well. When the expression is compiled | |
| 59 | as a POSIX basic or extended regex then Boost.Regex follows the POSIX standard | |
| 60 | leftmost longest rule for determining what matched. So if there is more than | |
| 61 | one possible match after considering the whole expression, it looks next | |
| 62 | at the first sub-expression and then the second sub-expression and so on. | |
| 63 | So... | |
| 64 | </p> | |
| 65 | <p> | |
| 66 | "(0*)([0-9]*)" against "00123" would produce $1 = "00" | |
| 67 | $2 = "123" | |
| 68 | </p> | |
| 69 | <p> | |
| 70 | where as | |
| 71 | </p> | |
| 72 | <p> | |
| 73 | "0*([0-9])*" against "00123" would produce $1 = "00123" | |
| 74 | </p> | |
| 75 | <p> | |
| 76 | If you think about it, had $1 only matched the "123", this would | |
| 77 | be "less good" than the match "00123" which is both further | |
| 78 | to the left and longer. If you want $1 to match only the "123" | |
| 79 | part, then you need to use something like: | |
| 80 | </p> | |
| 81 | <p> | |
| 82 | "0*([1-9][0-9]*)" | |
| 83 | </p> | |
| 84 | <p> | |
| 85 | as the expression. | |
| 86 | </p> | |
| 87 | <p> | |
| 88 | <span class="bold"><strong>Q.</strong></span> Why don't character ranges work properly | |
| 89 | (POSIX mode only)? | |
| 90 | </p> | |
| 91 | <p> | |
| 92 | <span class="bold"><strong>A.</strong></span> The POSIX standard specifies that character | |
| 93 | range expressions are locale sensitive - so for example the expression [A-Z] | |
| 94 | will match any collating element that collates between 'A' and 'Z'. That | |
| 95 | means that for most locales other than "C" or "POSIX", | |
| 96 | [A-Z] would match the single character 't' for example, which is not what | |
| 97 | most people expect - or at least not what most people have come to expect | |
| 98 | from regular expression engines. For this reason, the default behaviour of | |
| 99 | Boost.Regex (perl mode) is to turn locale sensitive collation off by not | |
| 100 | setting the <code class="computeroutput"><span class="identifier">regex_constants</span><span class="special">::</span><span class="identifier">collate</span></code> | |
| 101 | compile time flag. However if you set a non-default compile time flag - for | |
| 102 | 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>, | |
| 103 | then locale dependent collation will be enabled, this also applies to the | |
| 104 | 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> | |
| 105 | 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 | |
| 106 | "as if" the LC_COLLATE locale category were always "C", | |
| 107 | regardless of what its actually set to - end note]. | |
| 108 | </p> | |
| 109 | <p> | |
| 110 | <span class="bold"><strong>Q.</strong></span> Why are there no throw specifications | |
| 111 | on any of the functions? What exceptions can the library throw? | |
| 112 | </p> | |
| 113 | <p> | |
| 114 | <span class="bold"><strong>A.</strong></span> Not all compilers support (or honor) | |
| 115 | throw specifications, others support them but with reduced efficiency. Throw | |
| 116 | specifications may be added at a later date as compilers begin to handle | |
| 117 | this better. The library should throw only three types of exception: [boost::regex_error] | |
| 118 | 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 | |
| 119 | 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 | |
| 120 | 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 | |
| 121 | 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" | |
| 122 | search, or when a call to <code class="computeroutput"><span class="identifier">RegEx</span><span class="special">::</span><span class="identifier">GrepFiles</span></code> | |
| 123 | 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 | |
| 124 | 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 | |
| 125 | of the functions in this library. | |
| 126 | </p> | |
| 127 | <p> | |
| 128 | <span class="bold"><strong>Q.</strong></span> Why can't I use the "convenience" | |
| 129 | versions of regex_match / regex_search / regex_grep / regex_format / regex_merge? | |
| 130 | </p> | |
| 131 | <p> | |
| 132 | <span class="bold"><strong>A.</strong></span> These versions may or may not be available | |
| 133 | depending upon the capabilities of your compiler, the rules determining the | |
| 134 | format of these functions are quite complex - and only the versions visible | |
| 135 | to a standard compliant compiler are given in the help. To find out what | |
| 136 | your compiler supports, run <boost/regex.hpp> through your C++ pre-processor, | |
| 137 | and search the output file for the function that you are interested in. Note | |
| 138 | however, that very few current compilers still have problems with these overloaded | |
| 139 | functions. | |
| 140 | </p> | |
| 141 | </div> | |
| 142 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 143 | <td align="left"></td> | |
| 144 | <td align="right"><div class="copyright-footer">Copyright © 1998-2013 John Maddock<p> | |
| 145 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 146 | 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>) | |
| 147 | </p> | |
| 148 | </div></td> | |
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