Unicode and Boost.Regex

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## Unicode and Boost.Regex

There are two ways to use Boost.Regex with Unicode strings:

##### Rely on wchar_t

If your platform's `wchar_t` type can hold Unicode strings, and your platform's C/C++ runtime correctly handles wide character constants (when passed to `std::iswspace` `std::iswlower` etc), then you can use `boost::wregex` to process Unicode. However, there are several disadvantages to this approach:

- It's not portable: there's no guarantee on the width of `wchar_t`, or even whether the runtime treats wide characters as Unicode at all, most Windows compilers do so, but many Unix systems do not.
- There's no support for Unicode-specific character classes: `[[:Nd:]]`, `[[:Po:]]` etc.
- You can only search strings that are encoded as sequences of wide characters, it is not possible to search UTF-8, or even UTF-16 on many platforms.

##### Use a Unicode Aware Regular Expression Type.

If you have the ICU library, then Boost.Regex can be configured to make use of it, and provide a distinct regular expression type (boost::u32regex), that supports both Unicode specific character properties, and the searching of text that is encoded in either UTF-8, UTF-16, or UTF-32. See: ICU string class support.