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:

flag	effects on syntax	notes
icase	Case insensitive	Regular expressions match without regard to case.
nosubs	No sub-expressions	The match_results structure will not contain sub-expression matches.
optimize	Optimize matching	Matching efficiency is preferred over efficiency constructing regex objects.
collate	Locale sensitiveness	Character ranges, like "[a-b]", are affected by locale.
ECMAScript	ECMAScript grammar	The regular expression follows one of these grammars. One (and only one) of these six grammar flags needs to be set for the bitmask to have a valid value.
basic	Basic POSIX grammar
extended	Extended POSIX grammar
awk	Awk POSIX grammar
grep	Grep POSIX grammar
egrep	Egrep POSIX grammar

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. This constant has a value of zero**.
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). This constant has a value of zero**.
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.

** Constants with a value of zero are ignored if some other flag is set.

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.