31 Regular expressions library [re]

31.5 Namespace std​::​regex_­constants [re.const]

31.5.2 Bitmask type match_­flag_­type [re.matchflag]

namespace std::regex_constants {
  using match_flag_type = T2;
  inline constexpr match_flag_type match_default = {};
  inline constexpr match_flag_type match_not_bol = unspecified;
  inline constexpr match_flag_type match_not_eol = unspecified;
  inline constexpr match_flag_type match_not_bow = unspecified;
  inline constexpr match_flag_type match_not_eow = unspecified;
  inline constexpr match_flag_type match_any = unspecified;
  inline constexpr match_flag_type match_not_null = unspecified;
  inline constexpr match_flag_type match_continuous = unspecified;
  inline constexpr match_flag_type match_prev_avail = unspecified;
  inline constexpr match_flag_type format_default = {};
  inline constexpr match_flag_type format_sed = unspecified;
  inline constexpr match_flag_type format_no_copy = unspecified;
  inline constexpr match_flag_type format_first_only = unspecified;
}
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The type match_­flag_­type is an implementation-defined bitmask type. 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 131 for any bitmask elements set.

Table 131 — regex_­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 and iterators.
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.