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John Maddock
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<head>
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<title>Boost.Regex: FAQ</title>
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<p></p>
<table id="Table1" cellspacing="1" cellpadding="1" width="100%"
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<h3><a href="../../../index.htm"><img height="86" width="277" alt=
"C++ Boost" src="../../../c++boost.gif" border="0"></a></h3>
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<td width="353">
<h1 align="center">Boost.Regex</h1>
<h2 align="center">FAQ</h2>
</td>
<td width="50">
<h3><a href="index.html"><img height="45" width="43" alt=
"Boost.Regex Index" src="uarrow.gif" border="0"></a></h3>
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<br>
<br>
<hr>
<font color="#ff0000"><font color="#ff0000"></font></font>
<p><font color="#ff0000"><font color="#ff0000"><font color=
"#ff0000">&nbsp;Q. Why can't I use the "convenience" versions of
regex_match / regex_search / regex_grep / regex_format /
regex_merge?</font></font></font></p>
<p>A. 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 &lt;boost/regex.hpp&gt; through
your C++ pre-processor, and search the output file for the function
that you are interested in.<font color="#ff0000"><font color=
"#ff0000"></font></font></p>
<p><font color="#ff0000"><font color="#ff0000">Q. I can't get
regex++ to work with escape characters, what's going
on?</font></font></p>
<p>A. 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 regex++ expression compiler, so to pass
the regular expression \d+ to regex++, you need to embed "\\d+" in
your code. Likewise to match a literal backslash you will need to
embed "\\\\" in your code. <font color="#ff0000"></font></p>
<p><font color="#ff0000">Q. Why does using parenthesis in a POSIX
regular expression change the result of a match?</font></p>
<p>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>
<pre>
<head>
<title>Boost.Regex: FAQ</title>
<meta name="generator" content="HTML Tidy, see www.w3.org">
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" type="text/css" href="../../../boost.css">
</head>
<body>
<p></p>
<table id="Table1" cellspacing="1" cellpadding="1" width="100%" border="0">
<tr>
<td valign="top" width="300">
<h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../c++boost.gif" border="0"></a></h3>
</td>
<td width="353">
<h1 align="center">Boost.Regex</h1>
<h2 align="center">FAQ</h2>
</td>
<td width="50">
<h3><a href="index.html"><img height="45" width="43" alt="Boost.Regex Index" src="uarrow.gif" border="0"></a></h3>
</td>
</tr>
</table>
<br>
<br>
<hr>
<font color="#ff0000"><font color="#ff0000"></font></font>
<p><font color="#ff0000"><font color="#ff0000"><font color="#ff0000">&nbsp;Q. Why can't I
use the "convenience" versions of regex_match / regex_search / regex_grep /
regex_format / regex_merge?</font></font></font></p>
<p>A. 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
&lt;boost/regex.hpp&gt; through your C++ pre-processor, and search the output
file for the function that you are interested in.<font color="#ff0000"><font color="#ff0000"></font></font></p>
<p><font color="#ff0000"><font color="#ff0000">Q. I can't get regex++ to work with
escape characters, what's going on?</font></font></p>
<p>A. 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
regex++ expression compiler, so to pass the regular expression \d+ to regex++,
you need to embed "\\d+" in your code. Likewise to match a literal backslash
you will need to embed "\\\\" in your code. <font color="#ff0000"></font>
</p>
<p><font color="#ff0000">Q. Why does using parenthesis in a POSIX regular expression
change the result of a match?</font></p>
<p>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>
<pre>
"(0*)([0-9]*)" against "00123" would produce
$1 = "00"
$2 = "123"
</pre>
<p>where as</p>
<pre>
"0*([0-9)*" against "00123" would produce
<p>where as</p>
<pre>
"0*([0-9])*" against "00123" would produce
$1 = "00123"
</pre>
<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>
<pre>
<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>
<pre>
"0*([1-9][0-9]*)"
</pre>
<p>as the expression.</p>
<p><font color="#ff0000">Q. Why don't character ranges work
properly (POSIX mode only)?</font><br>
A. 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 regex_constants::collate
compile time flag. However if you set a non-default compile time
flag - for example regex_constants::extended or
regex_constants::basic, then locale dependent collation will be
enabled, this also applies to the POSIX API functions which use
either regex_constants::extended or regex_constants::basic
internally. <i>[Note - when regex_constants::nocollate in effect,
the library behaves "as if" the LC_COLLATE locale category were
always "C", regardless of what its actually set to - end
note</i>].</p>
<p><font color="#ff0000">Q. Why are there no throw specifications
on any of the functions? What exceptions can the library
throw?</font></p>
<p>A. 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::bad_expression can be thrown by basic_regex when
compiling a regular expression, std::runtime_error can be thrown
when a call to basic_regex::imbue tries to open a message catalogue
that doesn't exist, or when a call to regex_search or regex_match
results in an "everlasting" search,&nbsp;or when a call to
RegEx::GrepFiles or RegEx::FindFiles tries to open a file that
cannot be opened, finally std::bad_alloc can be thrown by just
about any of the functions in this library.</p>
<p></p>
<hr>
<p>as the expression.</p>
<p><font color="#ff0000">Q. Why don't character ranges work properly (POSIX mode
only)?</font><br>
A. 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 regex_constants::collate compile time flag.
However if you set a non-default compile time flag - for example
regex_constants::extended or regex_constants::basic, then locale dependent
collation will be enabled, this also applies to the POSIX API functions which
use either regex_constants::extended or regex_constants::basic internally. <i>[Note
- when regex_constants::nocollate in effect, the library behaves "as if" the
LC_COLLATE locale category were always "C", regardless of what its actually set
to - end note</i>].</p>
<p><font color="#ff0000">Q. Why are there no throw specifications on any of the
functions? What exceptions can the library throw?</font></p>
<p>A. 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::bad_expression can be thrown by basic_regex
when compiling a regular expression, std::runtime_error can be thrown when a
call to basic_regex::imbue tries to open a message catalogue that doesn't
exist, or when a call to regex_search or regex_match results in an
"everlasting" search,&nbsp;or when a call to RegEx::GrepFiles or
RegEx::FindFiles tries to open a file that cannot be opened, finally
std::bad_alloc can be thrown by just about any of the functions in this
library.</p>
<p></p>
<hr>
<p>Revised
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%d %B, %Y" startspan -->
24 Oct 2003
<!--webbot bot="Timestamp" endspan i-checksum="39359" --></p>
<p><i><EFBFBD> Copyright John Maddock&nbsp;1998-
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%Y" startspan -->
2003<!--webbot bot="Timestamp" endspan i-checksum="39359" --></i></p>
<!--webbot bot="Timestamp" S-Type="EDITED" S-Format="%Y" startspan --> 2003<!--webbot bot="Timestamp" endspan i-checksum="39359" --></i></p>
<P><I>Use, modification and distribution are subject to the Boost Software License,
Version 1.0. (See accompanying file <A href="../../../LICENSE_1_0.txt">LICENSE_1_0.txt</A>
or copy at <A href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>)</I></P>
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