std::regex_constants

This namespace declares three bitmask types used with elements of the regex library along with constant values of these types:

• syntax_option_type: Specifies the syntax in basic_regex objects.
• match_flag_type: Specifies options for regex matching operations.
• error_type: Provides error differentiation for the regex_error exception.

Each of these bitmask types may have been implemented in your library either as an enumerated type, an integer type, or a bitset. In any case, it can take one or more flags of that same type as value by combining them using the bitwise OR operator (|):


By providing these values under a specific namespace, you have the option to include this namespace (with using namespace std::regex_constants) directly in your code in order to refer to these values more easily independently on whether you include the more generic namespace std or not.

Bitmask type syntax_option_type

Aliased as member type flag_type in regex (and basic_regex), it is used in the construction or assignment of regex objects to specify the syntax used by the object. The possible values it can take are:

Bitmask type match_flag_type

Used as a parameter to functions regex_match, regex_search and regex_replace and also as a parameter to the constructors of regex_iterator and regex_token_iterator.

flag	effects	notes
match_default	Default	Default matching behavior.**.
match_not_bol	Not Beginning-Of-Line	The first character is not considered a beginning of line ("^" does not match).
match_not_eol	Not End-Of-Line	The last character is not considered an end of line ("$" does not match).
match_not_bow	Not Beginning-Of-Word	The escape sequence "\b" does not match as a beginning-of-word.
match_not_eow	Not End-Of-Word	The escape sequence "\b" does not match as an end-of-word.
match_any	Any match	Any match is acceptable if more than one match is possible.
match_not_null	Not null	Empty sequences do not match.
match_continuous	Continuous	The expression must match a sub-sequence that begins at the first character. Sub-sequences must begin at the first character to match.
match_prev_avail	Previous Available	One or more characters exist before the first one. (match_not_bol and match_not_bow are ignored)
format_default	Default formatting	Uses the standard formatting rules to replace matches (those used by ECMAScript's replace method).**.
format_sed	sed formatting	Uses the same rules as the sed utility in POSIX to replace matches.
format_no_copy	No copy	The sections in the target sequence that do not match the regular expression are not copied when replacing matches.
format_first_only	First only	Only the first occurrence of a regular expression is replaced.

** These constants have an empty bitmask value and are ignored when combined with some other flag value.

Bitmask type error_type

Used as by the regex_error to identify the kind of error that threw the exception:

flag	error
error_collate	The expression contained an invalid collating element name.
error_ctype	The expression contained an invalid character class name.
error_escape	The expression contained an invalid escaped character, or a trailing escape.
error_backref	The expression contained an invalid back reference.
error_brack	The expression contained mismatched brackets ([ and ]).
error_paren	The expression contained mismatched parentheses (( and )).
error_brace	The expression contained mismatched braces ({ and }).
error_badbrace	The expression contained an invalid range between braces ({ and }).
error_range	The expression contained an invalid character range.
error_space	There was insufficient memory to convert the expression info a finite state machine.
error_badrepeat	The expression contained a repeat specifier (one of *?+{) that was not preceded by a valid regular expression.
error_complexity	The complexity of an attempted match against a regular expression exceeded a pre-set level.
error_stack	There was insufficient memory to determine whether the regular expression could match the specified character sequence.