30 Regular expressions library [re]

30.9 Class template match_­results [re.results]

30.9.1 General [re.results.general]

Class template match_­results denotes a collection of character sequences representing the result of a regular expression match.
Storage for the collection is allocated and freed as necessary by the member functions of class template match_­results.
The class template match_­results meets the requirements of an allocator-aware container and of a sequence container ([container.requirements.general], [sequence.reqmts]) except that only copy assignment, move assignment, and operations defined for const-qualified sequence containers are supported and that the semantics of the comparison operator functions are different from those required for a container.
A default-constructed match_­results object has no fully established result state.
A match result is ready when, as a consequence of a completed regular expression match modifying such an object, its result state becomes fully established.
The effects of calling most member functions from a match_­results object that is not ready are undefined.
The sub_­match object stored at index 0 represents sub-expression 0, i.e., the whole match.
In this case the sub_­match member matched is always true.
The sub_­match object stored at index n denotes what matched the marked sub-expression n within the matched expression.
If the sub-expression n participated in a regular expression match then the sub_­match member matched evaluates to true, and members first and second denote the range of characters [first, second) which formed that match.
Otherwise matched is false, and members first and second point to the end of the sequence that was searched.
[Note 1:
The sub_­match objects representing different sub-expressions that did not participate in a regular expression match need not be distinct.
— end note]
namespace std { template<class BidirectionalIterator, class Allocator = allocator<sub_match<BidirectionalIterator>>> class match_results { public: using value_type = sub_match<BidirectionalIterator>; using const_reference = const value_type&; using reference = value_type&; using const_iterator = implementation-defined; using iterator = const_iterator; using difference_type = typename iterator_traits<BidirectionalIterator>::difference_type; using size_type = typename allocator_traits<Allocator>::size_type; using allocator_type = Allocator; using char_type = typename iterator_traits<BidirectionalIterator>::value_type; using string_type = basic_string<char_type>; // [re.results.const], construct/copy/destroy match_results() : match_results(Allocator()) {} explicit match_results(const Allocator&); match_results(const match_results& m); match_results(match_results&& m) noexcept; match_results& operator=(const match_results& m); match_results& operator=(match_results&& m); ~match_results(); // [re.results.state], state bool ready() const; // [re.results.size], size size_type size() const; size_type max_size() const; [[nodiscard]] bool empty() const; // [re.results.acc], element access difference_type length(size_type sub = 0) const; difference_type position(size_type sub = 0) const; string_type str(size_type sub = 0) const; const_reference operator[](size_type n) const; const_reference prefix() const; const_reference suffix() const; const_iterator begin() const; const_iterator end() const; const_iterator cbegin() const; const_iterator cend() const; // [re.results.form], format template<class OutputIter> OutputIter format(OutputIter out, const char_type* fmt_first, const char_type* fmt_last, regex_constants::match_flag_type flags = regex_constants::format_default) const; template<class OutputIter, class ST, class SA> OutputIter format(OutputIter out, const basic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const; template<class ST, class SA> basic_string<char_type, ST, SA> format(const basic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const; string_type format(const char_type* fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const; // [re.results.all], allocator allocator_type get_allocator() const; // [re.results.swap], swap void swap(match_results& that); }; }

30.9.2 Constructors [re.results.const]

explicit match_results(const Allocator& a);
Postconditions: ready() returns false.
size() returns 0.
match_results(match_results&& m) noexcept;
Effects: The stored Allocator value is move constructed from m.get_­allocator().
Postconditions: As specified in Table 142.
match_results& operator=(const match_results& m);
Postconditions: As specified in Table 142.
match_results& operator=(match_results&& m);
Postconditions: As specified in Table 142.
Table 142: match_­results assignment operator effects [tab:re.results.const]
Element
Value
ready()
m.ready()
size()
m.size()
str(n)
m.str(n) for all integers n < m.size()
prefix()
m.prefix()
suffix()
m.suffix()
(*this)[n]
m[n] for all integers n < m.size()
length(n)
m.length(n) for all integers n < m.size()
position(n)
m.position(n) for all integers n < m.size()

30.9.3 State [re.results.state]

bool ready() const;
Returns: true if *this has a fully established result state, otherwise false.

30.9.4 Size [re.results.size]

size_type size() const;
Returns: One plus the number of marked sub-expressions in the regular expression that was matched if *this represents the result of a successful match.
Otherwise returns 0.
[Note 1:
The state of a match_­results object can be modified only by passing that object to regex_­match or regex_­search.
Subclauses [re.alg.match] and [re.alg.search] specify the effects of those algorithms on their match_­results arguments.
— end note]
size_type max_size() const;
Returns: The maximum number of sub_­match elements that can be stored in *this.
[[nodiscard]] bool empty() const;
Returns: size() == 0.

30.9.5 Element access [re.results.acc]

difference_type length(size_type sub = 0) const;
Preconditions: ready() == true.
Returns: (*this)[sub].length().
difference_type position(size_type sub = 0) const;
Preconditions: ready() == true.
Returns: The distance from the start of the target sequence to (*this)[sub].first.
string_type str(size_type sub = 0) const;
Preconditions: ready() == true.
Returns: string_­type((*this)[sub]).
const_reference operator[](size_type n) const;
Preconditions: ready() == true.
Returns: A reference to the sub_­match object representing the character sequence that matched marked sub-expression n.
If n == 0 then returns a reference to a sub_­match object representing the character sequence that matched the whole regular expression.
If n >= size() then returns a sub_­match object representing an unmatched sub-expression.
const_reference prefix() const;
Preconditions: ready() == true.
Returns: A reference to the sub_­match object representing the character sequence from the start of the string being matched/searched to the start of the match found.
const_reference suffix() const;
Preconditions: ready() == true.
Returns: A reference to the sub_­match object representing the character sequence from the end of the match found to the end of the string being matched/searched.
const_iterator begin() const; const_iterator cbegin() const;
Returns: A starting iterator that enumerates over all the sub-expressions stored in *this.
const_iterator end() const; const_iterator cend() const;
Returns: A terminating iterator that enumerates over all the sub-expressions stored in *this.

30.9.6 Formatting [re.results.form]

template<class OutputIter> OutputIter format( OutputIter out, const char_type* fmt_first, const char_type* fmt_last, regex_constants::match_flag_type flags = regex_constants::format_default) const;
Preconditions: ready() == true and OutputIter meets the requirements for a Cpp17OutputIterator ([output.iterators]).
Effects: Copies the character sequence [fmt_­first, fmt_­last) to OutputIter out.
Replaces each format specifier or escape sequence in the copied range with either the character(s) it represents or the sequence of characters within *this to which it refers.
The bitmasks specified in flags determine which format specifiers and escape sequences are recognized.
Returns: out.
template<class OutputIter, class ST, class SA> OutputIter format( OutputIter out, const basic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const;
Effects: Equivalent to: return format(out, fmt.data(), fmt.data() + fmt.size(), flags);
template<class ST, class SA> basic_string<char_type, ST, SA> format( const basic_string<char_type, ST, SA>& fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const;
Preconditions: ready() == true.
Effects: Constructs an empty string result of type basic_­string<char_­type, ST, SA> and calls: format(back_inserter(result), fmt, flags);
Returns: result.
string_type format( const char_type* fmt, regex_constants::match_flag_type flags = regex_constants::format_default) const;
Preconditions: ready() == true.
Effects: Constructs an empty string result of type string_­type and calls: format(back_inserter(result), fmt, fmt + char_traits<char_type>::length(fmt), flags);
Returns: result.

30.9.7 Allocator [re.results.all]

allocator_type get_allocator() const;
Returns: A copy of the Allocator that was passed to the object's constructor or, if that allocator has been replaced, a copy of the most recent replacement.

30.9.8 Swap [re.results.swap]

void swap(match_results& that);
Effects: Swaps the contents of the two sequences.
Postconditions: *this contains the sequence of matched sub-expressions that were in that, that contains the sequence of matched sub-expressions that were in *this.
Complexity: Constant time.
template<class BidirectionalIterator, class Allocator> void swap(match_results<BidirectionalIterator, Allocator>& m1, match_results<BidirectionalIterator, Allocator>& m2);
Effects: As if by m1.swap(m2).

30.9.9 Non-member functions [re.results.nonmember]

template<class BidirectionalIterator, class Allocator> bool operator==(const match_results<BidirectionalIterator, Allocator>& m1, const match_results<BidirectionalIterator, Allocator>& m2);
Returns: true if neither match result is ready, false if one match result is ready and the other is not.
If both match results are ready, returns true only if:
  • m1.empty() && m2.empty(), or
  • !m1.empty() && !m2.empty(), and the following conditions are satisfied:
    • m1.prefix() == m2.prefix(),
    • m1.size() == m2.size() && equal(m1.begin(), m1.end(), m2.begin()), and
    • m1.suffix() == m2.suffix().
[Note 1:
The algorithm equal is defined in [algorithms].
— end note]