Boost.Regex

Boost.regex provides extensive support for run-time localization, the localization model used can be split into two parts: front-end and back-end.

Front-end localization deals with everything which the user sees - error messages, and the regular expression syntax itself. For example a French application could change [[:word:]] to [[:mot:]] and \w to \m. Modifying the front end locale requires active support from the developer, by providing the library with a message catalogue to load, containing the localized strings. Front-end locale is affected by the LC_MESSAGES category only.

Back-end localization deals with everything that occurs after the expression has been parsed - in other words everything that the user does not see or interact with directly. It deals with case conversion, collation, and character class membership. The back-end locale does not require any intervention from the developer - the library will acquire all the information it requires for the current locale from the underlying operating system / run time library. This means that if the program user does not interact with regular expressions directly - for example if the expressions are embedded in your C++ code - then no explicit localization is required, as the library will take care of everything for you. For example embedding the expression [[:word:]]+ in your code will always match a whole word, if the program is run on a machine with, for example, a Greek locale, then it will still match a whole word, but in Greek characters rather than Latin ones. The back-end locale is affected by the LC_TYPE and LC_COLLATE categories.

There are three separate localization mechanisms supported by boost.regex:

Win32 localization model.

This is the default model when the library is compiled under Win32, and is encapsulated by the traits class w32_regex_traits. When this model is in effect each basic_regex object gets it's own LCID, by default this is the users default setting as returned by GetUserDefaultLCID, but you can call imbue on the basic_regex object to set it's locale to some other LCID if you wish. All the settings used by boost.regex are acquired directly from the operating system bypassing the C run time library. Front-end localization requires a resource dll, containing a string table with the user-defined strings. The traits class exports the function:

static std::string set_message_catalogue(const std::string& s);

which needs to be called with a string identifying the name of the resource dll, before your code compiles any regular expressions (but not necessarily before you construct any basic_regex instances):

boost::w32_regex_traits<char>::set_message_catalogue("mydll.dll");

The library provides full Unicode support under NT, under Windows 9x the library degrades gracefully - characters 0 to 255 are supported, the remainder are treated as "unknown" graphic characters.

C localization model.

This model has been deprecated in favor of the C++ localoe for all non-Windows compilers that support it.  This locale is encapsulated by the traits class c_regex_traits, Win32 users can force this model to take effect by defining the pre-processor symbol BOOST_REGEX_USE_C_LOCALE. When this model is in effect there is a single global locale, as set by setlocale. All settings are acquired from your run time library, consequently Unicode support is dependent upon your run time library implementation.

Front end localization is not supported.

Note that calling setlocale invalidates all compiled regular expressions, calling setlocale(LC_ALL, "C") will make this library behave equivalent to most traditional regular expression libraries including version 1 of this library.

C++ localization model.

This model is the default for non-Windows compilers.

When this model is in effect each instance of basic_regex<> has its own instance of std::locale, class basic_regex<> also has a member function imbue which allows the locale for the expression to be set on a per-instance basis. Front end localization requires a POSIX message catalogue, which will be loaded via the std::messages facet of the expression's locale, the traits class exports the symbol:

static std::string set_message_catalogue(const std::string& s);

which needs to be called with a string identifying the name of the message catalogue, before your code compiles any regular expressions (but not necessarily before you construct any basic_regex instances):

boost::cpp_regex_traits<char>::set_message_catalogue("mycatalogue");

Note that calling basic_regex<>::imbue will invalidate any expression currently compiled in that instance of basic_regex<>.

Finally note that if you build the library with a non-default localization model, then the appropriate pre-processor symbol (BOOST_REGEX_USE_C_LOCALE or BOOST_REGEX_USE_CPP_LOCALE) must be defined both when you build the support library, and when you include <boost/regex.hpp> or <boost/cregex.hpp> in your code. The best way to ensure this is to add the #define to <boost/regex/user.hpp>.

Providing a message catalogue:

In order to localize the front end of the library, you need to provide the library with the appropriate message strings contained either in a resource dll's string table (Win32 model), or a POSIX message catalogue (C++ models). In the latter case the messages must appear in message set zero of the catalogue. The messages and their id's are as follows:
 

Custom error messages are loaded as follows: 

Custom character class names are loaded as followed: 

Finally, custom collating element names are loaded starting from message id 400, and terminating when the first load thereafter fails. Each message looks something like: "tagname string" where tagname is the name used inside [[.tagname.]] and string is the actual text of the collating element. Note that the value of collating element [[.zero.]] is used for the conversion of strings to numbers - if you replace this with another value then that will be used for string parsing - for example use the Unicode character 0x0660 for [[.zero.]] if you want to use Unicode Arabic-Indic digits in your regular expressions in place of Latin digits.

Note that the POSIX defined names for character classes and collating elements are always available - even if custom names are defined, in contrast, custom error messages, and custom syntax messages replace the default ones.

Revised  26 June 2004 

© Copyright John Maddock 1998- 2004

Use, modification and distribution are subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)