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