28 Regular expressions library [re]

28.5 Namespace std::regex_constants [re.const]

28.5.2 Bitmask type regex_constants::match_flag_type [re.matchflag]

namespace std {
  namespace regex_constants{
    typedef T2 match_flag_type;
    constexpr match_flag_type match_default = {};
    constexpr match_flag_type match_not_bol = unspecified;
    constexpr match_flag_type match_not_eol = unspecified;
    constexpr match_flag_type match_not_bow = unspecified;
    constexpr match_flag_type match_not_eow = unspecified;
    constexpr match_flag_type match_any = unspecified;
    constexpr match_flag_type match_not_null = unspecified;
    constexpr match_flag_type match_continuous = unspecified;
    constexpr match_flag_type match_prev_avail = unspecified;
    constexpr match_flag_type format_default = {};
    constexpr match_flag_type format_sed = unspecified;
    constexpr match_flag_type format_no_copy = unspecified;
    constexpr match_flag_type format_first_only = unspecified;
  }
}

The type regex_constants::match_flag_type is an implementation-defined bitmask type ([bitmask.types]). The constants of that type, except for match_default and format_default, are bitmask elements. The match_default and format_default constants are empty bitmasks. Matching a regular expression against a sequence of characters [first,last) proceeds according to the rules of the grammar specified for the regular expression object, modified according to the effects listed in Table [tab:re:matchflag] for any bitmask elements set.

Table 139regex_constants::match_flag_type effects when obtaining a match against a character container sequence [first,last).
ElementEffect(s) if set
match_not_bol The first character in the sequence [first,last) shall be treated as though it is not at the beginning of a line, so the character ^ in the regular expression shall not match [first,first).
match_not_eol The last character in the sequence [first,last) shall be treated as though it is not at the end of a line, so the character "$" in the regular expression shall not match [last,last).
match_not_bow The expression "\\b" shall not match the sub-sequence [first,first).
match_not_eow The expression "\\b" shall not match the sub-sequence [last,last).
match_any If more than one match is possible then any match is an acceptable result.
match_not_null The expression shall not match an empty sequence.
match_continuous The expression shall only match a sub-sequence that begins at first.
match_prev_avail --first is a valid iterator position. When this flag is set the flags match_not_bol and match_not_bow shall be ignored by the regular expression algorithms [re.alg] and iterators [re.iter].
format_default When a regular expression match is to be replaced by a new string, the new string shall be constructed using the rules used by the ECMAScript replace function in ECMA-262, part 15.5.4.11 String.prototype.replace. In addition, during search and replace operations all non-overlapping occurrences of the regular expression shall be located and replaced, and sections of the input that did not match the expression shall be copied unchanged to the output string.
format_sed When a regular expression match is to be replaced by a new string, the new string shall be constructed using the rules used by the sed utility in POSIX.
format_no_copy During a search and replace operation, sections of the character container sequence being searched that do not match the regular expression shall not be copied to the output string.
format_first_only When specified during a search and replace operation, only the first occurrence of the regular expression shall be replaced.