| [/ |
| Copyright 2006-2007 John Maddock. |
| 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:captures Understanding Marked Sub-Expressions and Captures] |
| |
| Captures are the iterator ranges that are "captured" by marked |
| sub-expressions as a regular expression gets matched. Each marked |
| sub-expression can result in more than one capture, if it is matched |
| more than once. This document explains how captures and marked |
| sub-expressions in Boost.Regex are represented and accessed. |
| |
| [h4 Marked sub-expressions] |
| |
| Every time a Perl regular expression contains a parenthesis group `()`, it |
| spits out an extra field, known as a marked sub-expression, |
| for example the expression: |
| |
| [pre (\w+)\W+(\w+)] |
| |
| Has two marked sub-expressions (known as $1 and $2 respectively), in |
| addition the complete match is known as $&, everything before the |
| first match as $\`, and everything after the match as $'. So |
| if the above expression is searched for within `"@abc def--"`, then we obtain: |
| |
| [table |
| [[Sub-expression][Text found]] |
| [[$\`]["@"]] |
| [[$&]["abc def"]] |
| [[$1]["abc"]] |
| [[$2]["def"]] |
| [[$']["--"]] |
| ] |
| |
| In Boost.Regex all these are accessible via the [match_results] class that |
| gets filled in when calling one of the regular expression matching algorithms |
| ([regex_search], [regex_match], or [regex_iterator]). So given: |
| |
| boost::match_results<IteratorType> m; |
| |
| The Perl and Boost.Regex equivalents are as follows: |
| |
| [table |
| [[Perl][Boost.Regex]] |
| [[$\`][`m.prefix()`]] |
| [[$&][`m[0]`]] |
| [[$n][`m[n]`]] |
| [[$\'][`m.suffix()`]] |
| ] |
| |
| In Boost.Regex each sub-expression match is represented by a [sub_match] object, |
| this is basically just a pair of iterators denoting the start and end |
| position of the sub-expression match, but there are some additional |
| operators provided so that objects of type [sub_match] behave a lot like a |
| `std::basic_string`: for example they are implicitly convertible to a |
| `basic_string`, they can be compared to a string, added to a string, or |
| streamed out to an output stream. |
| |
| [h4 Unmatched Sub-Expressions] |
| |
| When a regular expression match is found there is no need for all of the |
| marked sub-expressions to have participated in the match, for example the expression: |
| |
| [pre (abc)|(def)] |
| |
| can match either $1 or $2, but never both at the same time. In Boost.Regex |
| you can determine which sub-expressions matched by accessing the |
| `sub_match::matched` data member. |
| |
| [h4 Repeated Captures] |
| |
| When a marked sub-expression is repeated, then the sub-expression gets |
| "captured" multiple times, however normally only the final capture is available, |
| for example if |
| |
| [pre (?:(\w+)\W+)+] |
| |
| is matched against |
| |
| [pre one fine day] |
| |
| Then $1 will contain the string "day", and all the previous captures will have |
| been forgotten. |
| |
| However, Boost.Regex has an experimental feature that allows all the capture |
| information to be retained - this is accessed either via the |
| `match_results::captures` member function or the `sub_match::captures` member |
| function. These functions return a container that contains a sequence of all |
| the captures obtained during the regular expression matching. The following |
| example program shows how this information may be used: |
| |
| #include <boost/regex.hpp> |
| #include <iostream> |
| |
| void print_captures(const std::string& regx, const std::string& text) |
| { |
| boost::regex e(regx); |
| boost::smatch what; |
| std::cout << "Expression: \"" << regx << "\"\n"; |
| std::cout << "Text: \"" << text << "\"\n"; |
| if(boost::regex_match(text, what, e, boost::match_extra)) |
| { |
| unsigned i, j; |
| std::cout << "** Match found **\n Sub-Expressions:\n"; |
| for(i = 0; i < what.size(); ++i) |
| std::cout << " $" << i << " = \"" << what[i] << "\"\n"; |
| std::cout << " Captures:\n"; |
| for(i = 0; i < what.size(); ++i) |
| { |
| std::cout << " $" << i << " = {"; |
| for(j = 0; j < what.captures(i).size(); ++j) |
| { |
| if(j) |
| std::cout << ", "; |
| else |
| std::cout << " "; |
| std::cout << "\"" << what.captures(i)[j] << "\""; |
| } |
| std::cout << " }\n"; |
| } |
| } |
| else |
| { |
| std::cout << "** No Match found **\n"; |
| } |
| } |
| |
| int main(int , char* []) |
| { |
| print_captures("(([[:lower:]]+)|([[:upper:]]+))+", "aBBcccDDDDDeeeeeeee"); |
| print_captures("(.*)bar|(.*)bah", "abcbar"); |
| print_captures("(.*)bar|(.*)bah", "abcbah"); |
| print_captures("^(?:(\\w+)|(?>\\W+))*$", |
| "now is the time for all good men to come to the aid of the party"); |
| return 0; |
| } |
| |
| Which produces the following output: |
| |
| [pre |
| Expression: "(([[:lower:\]\]+)|([[:upper:\]\]+))+" |
| Text: "aBBcccDDDDDeeeeeeee" |
| '''**''' Match found '''**''' |
| Sub-Expressions: |
| $0 = "aBBcccDDDDDeeeeeeee" |
| $1 = "eeeeeeee" |
| $2 = "eeeeeeee" |
| $3 = "DDDDD" |
| Captures: |
| $0 = { "aBBcccDDDDDeeeeeeee" } |
| $1 = { "a", "BB", "ccc", "DDDDD", "eeeeeeee" } |
| $2 = { "a", "ccc", "eeeeeeee" } |
| $3 = { "BB", "DDDDD" } |
| Expression: "(.'''*''')bar|(.'''*''')bah" |
| Text: "abcbar" |
| '''**''' Match found '''**''' |
| Sub-Expressions: |
| $0 = "abcbar" |
| $1 = "abc" |
| $2 = "" |
| Captures: |
| $0 = { "abcbar" } |
| $1 = { "abc" } |
| $2 = { } |
| Expression: "(.'''*''')bar|(.'''*''')bah" |
| Text: "abcbah" |
| '''**''' Match found '''**''' |
| Sub-Expressions: |
| $0 = "abcbah" |
| $1 = "" |
| $2 = "abc" |
| Captures: |
| $0 = { "abcbah" } |
| $1 = { } |
| $2 = { "abc" } |
| Expression: "^(?:(\w+)|(?>\W+))'''*$'''" |
| Text: "now is the time for all good men to come to the aid of the party" |
| '''**''' Match found '''**''' |
| Sub-Expressions: |
| $0 = "now is the time for all good men to come to the aid of the party" |
| $1 = "party" |
| Captures: |
| $0 = { "now is the time for all good men to come to the aid of the party" } |
| $1 = { "now", "is", "the", "time", "for", "all", "good", "men", "to", |
| "come", "to", "the", "aid", "of", "the", "party" } |
| ] |
| |
| Unfortunately enabling this feature has an impact on performance |
| (even if you don't use it), and a much bigger impact if you do use it, |
| therefore to use this feature you need to: |
| |
| * Define BOOST_REGEX_MATCH_EXTRA for all translation units including the library source (the best way to do this is to uncomment this define in boost/regex/user.hpp and then rebuild everything. |
| * Pass the match_extra flag to the particular algorithms where you actually need the captures information (regex_search, regex_match, or regex_iterator). |
| |
| [endsect] |
| |