namespace std::regex_constants {
using syntax_option_type = T1;
constexpr syntax_option_type icase = unspecified;
constexpr syntax_option_type nosubs = unspecified;
constexpr syntax_option_type optimize = unspecified;
constexpr syntax_option_type collate = unspecified;
constexpr syntax_option_type ECMAScript = unspecified;
constexpr syntax_option_type basic = unspecified;
constexpr syntax_option_type extended = unspecified;
constexpr syntax_option_type awk = unspecified;
constexpr syntax_option_type grep = unspecified;
constexpr syntax_option_type egrep = unspecified;
constexpr syntax_option_type multiline = unspecified;
}
The type syntax_option_type is an implementation-defined bitmask type ([bitmask.types]). Setting its elements has the effects listed in Table [tab:re:syntaxoption]. A valid value of type syntax_option_type shall have at most one of the grammar elements ECMAScript, basic, extended, awk, grep, egrep, set. If no grammar element is set, the default grammar is ECMAScript.
| Element | Effect(s) if set |
| icase | Specifies that matching of regular expressions against a character container sequence shall be performed without regard to case. |
| nosubs | Specifies that no sub-expressions shall be considered to be marked, so that when a regular expression is matched against a character container sequence, no sub-expression matches shall be stored in the supplied match_results structure. |
| optimize | Specifies that the regular expression engine should pay more attention to the speed with which regular expressions are matched, and less to the speed with which regular expression objects are constructed. Otherwise it has no detectable effect on the program output. |
| collate | Specifies that character ranges of the form "[a-b]" shall be locale sensitive. |
| ECMAScript | Specifies that the grammar recognized by the regular expression engine shall be that used by ECMAScript in ECMA-262, as modified in [re.grammar]. |
| basic | Specifies that the grammar recognized by the regular expression engine shall be that used by basic regular expressions in POSIX, Base Definitions and Headers, Section 9, Regular Expressions. |
| extended | Specifies that the grammar recognized by the regular expression engine shall be that used by extended regular expressions in POSIX, Base Definitions and Headers, Section 9, Regular Expressions. |
| awk | Specifies that the grammar recognized by the regular expression engine shall be that used by the utility awk in POSIX. |
| grep | Specifies that the grammar recognized by the regular expression engine shall be that used by the utility grep in POSIX. |
| egrep | Specifies that the grammar recognized by the regular expression engine shall be that used by the utility grep when given the -E option in POSIX. |
| multiline | Specifies that ^ shall match the beginning of a line and $ shall match the end of a line, if the ECMAScript engine is selected. |