...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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.
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:
(\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:
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:
Perl |
Boost.Regex |
---|---|
$` |
|
$& |
|
$n |
|
$' |
|
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.
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:
(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.
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
(?:(\w+)\W+)+
is matched against
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:
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: