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- [/
- / Copyright (c) 2009 Eric Niebler
- /
- / Distributed under 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)
- /]
- [section Named Captures]
- [h2 Overview]
- For complicated regular expressions, dealing with numbered captures can be a
- pain. Counting left parentheses to figure out which capture to reference is
- no fun. Less fun is the fact that merely editing a regular expression could
- cause a capture to be assigned a new number, invaliding code that refers back
- to it by the old number.
- Other regular expression engines solve this problem with a feature called
- /named captures/. This feature allows you to assign a name to a capture, and
- to refer back to the capture by name rather by number. Xpressive also supports
- named captures, both in dynamic and in static regexes.
- [h2 Dynamic Named Captures]
- For dynamic regular expressions, xpressive follows the lead of other popular
- regex engines with the syntax of named captures. You can create a named capture
- with `"(?P<xxx>...)"` and refer back to that capture with `"(?P=xxx)"`. Here,
- for instance, is a regular expression that creates a named capture and refers
- back to it:
- // Create a named capture called "char" that matches a single
- // character and refer back to that capture by name.
- sregex rx = sregex::compile("(?P<char>.)(?P=char)");
- The effect of the above regular expression is to find the first doubled
- character.
- Once you have executed a match or search operation using a regex with named
- captures, you can access the named capture through the _match_results_ object
- using the capture's name.
- std::string str("tweet");
- sregex rx = sregex::compile("(?P<char>.)(?P=char)");
- smatch what;
- if(regex_search(str, what, rx))
- {
- std::cout << "char = " << what["char"] << std::endl;
- }
- The above code displays:
- [pre
- char = e
- ]
- You can also refer back to a named capture from within a substitution string.
- The syntax for that is `"\\g<xxx>"`. Below is some code that demonstrates how
- to use named captures when doing string substitution.
- std::string str("tweet");
- sregex rx = sregex::compile("(?P<char>.)(?P=char)");
- str = regex_replace(str, rx, "**\\g<char>**", regex_constants::format_perl);
- std::cout << str << std::endl;
- Notice that you have to specify `format_perl` when using named captures. Only
- the perl syntax recognizes the `"\\g<xxx>"` syntax. The above code displays:
- [pre
- tw\*\*e\*\*t
- ]
- [h2 Static Named Captures]
- If you're using static regular expressions, creating and using named
- captures is even easier. You can use the _mark_tag_ type to create
- a variable that you can use like [globalref boost::xpressive::s1 `s1`],
- [globalref boost::xpressive::s1 `s2`] and friends, but with a name
- that is more meaningful. Below is how the above example would look
- using static regexes:
- mark_tag char_(1); // char_ is now a synonym for s1
- sregex rx = (char_= _) >> char_;
- After a match operation, you can use the `mark_tag` to index into the
- _match_results_ to access the named capture:
- std::string str("tweet");
- mark_tag char_(1);
- sregex rx = (char_= _) >> char_;
- smatch what;
- if(regex_search(str, what, rx))
- {
- std::cout << what[char_] << std::endl;
- }
- The above code displays:
- [pre
- char = e
- ]
- When doing string substitutions with _regex_replace_, you can use named
- captures to create /format expressions/ as below:
- std::string str("tweet");
- mark_tag char_(1);
- sregex rx = (char_= _) >> char_;
- str = regex_replace(str, rx, "**" + char_ + "**");
- std::cout << str << std::endl;
- The above code displays:
- [pre
- tw\*\*e\*\*t
- ]
- [note You need to include [^<boost/xpressive/regex_actions.hpp>] to
- use format expressions.]
- [endsect]
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