1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>FAQ</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Boost.Regex 5.1.4"> 8<link rel="up" href="../background.html" title="Background Information"> 9<link rel="prev" href="futher.html" title="References and Further Information"> 10<link rel="next" href="performance.html" title="Performance"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="futher.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../background.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="performance.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h3 class="title"> 27<a name="boost_regex.background.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> 149</tr></table> 150<hr> 151<div class="spirit-nav"> 152<a accesskey="p" href="futher.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../background.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="performance.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 153</div> 154</body> 155</html> 156
