65 lines
2.9 KiB
Plaintext
65 lines
2.9 KiB
Plaintext
[/
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/ Copyright (c) 2008 Eric Niebler
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/
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/ Distributed under the Boost Software License, Version 1.0. (See accompanying
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/ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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/]
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[section Dynamic Regexes]
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[h2 Overview]
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Static regexes are dandy, but sometimes you need something a bit more ... dynamic. Imagine you are developing
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a text editor with a regex search/replace feature. You need to accept a regular expression from the end user
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as input at run-time. There should be a way to parse a string into a regular expression. That's what xpressive's
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dynamic regexes are for. They are built from the same core components as their static counterparts, but they
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are late-bound so you can specify them at run-time.
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[h2 Construction and Assignment]
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There are two ways to create a dynamic regex: with the _regex_compile_
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function or with the _regex_compiler_ class template. Use _regex_compile_
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if you want the default locale. Use _regex_compiler_ if you need to
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specify a different locale. In the section on
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[link boost_xpressive.user_s_guide.grammars_and_nested_matches regex grammars],
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we'll see another use for _regex_compiler_.
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Here is an example of using `basic_regex<>::compile()`:
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sregex re = sregex::compile( "this|that", regex_constants::icase );
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Here is the same example using _regex_compiler_:
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sregex_compiler compiler;
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sregex re = compiler.compile( "this|that", regex_constants::icase );
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_regex_compile_ is implemented in terms of _regex_compiler_.
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[h2 Dynamic xpressive Syntax]
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Since the dynamic syntax is not constrained by the rules for valid C++ expressions, we are free to use familiar
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syntax for dynamic regexes. For this reason, the syntax used by xpressive for dynamic regexes follows the
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lead set by John Maddock's [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1429.htm proposal]
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to add regular expressions to the Standard Library. It is essentially the syntax standardized by
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[@http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf ECMAScript], with minor changes
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in support of internationalization.
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Since the syntax is documented exhaustively elsewhere, I will simply refer you to the existing standards, rather
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than duplicate the specification here.
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[h2 Internationalization]
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As with static regexes, dynamic regexes support internationalization by allowing you to specify a different
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`std::locale`. To do this, you must use _regex_compiler_. The _regex_compiler_ class has an `imbue()` function.
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After you have imbued a _regex_compiler_ object with a custom `std::locale`, all regex objects compiled by
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that _regex_compiler_ will use that locale. For example:
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std::locale my_locale = /* initialize your locale object here */;
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sregex_compiler compiler;
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compiler.imbue( my_locale );
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sregex re = compiler.compile( "\\w+|\\d+" );
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This regex will use `my_locale` when evaluating the intrinsic character sets `"\\w"` and `"\\d"`.
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[endsect]
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