28 Text processing library [text]

28.6 Regular expressions library [re]

28.6.4 Namespace std::regex_constants [re.const]

28.6.4.1 General [re.const.general]

The namespace std::regex_constants holds symbolic constants used by the regular expression library.

This namespace provides three types, syntax_option_type, match_flag_type, and error_type, along with several constants of these types.

28.6.4.2 Bitmask type syntax_option_type [re.synopt]

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namespace std::regex_constants { using syntax_option_type = T1; inline constexpr syntax_option_type icase = unspecified; inline constexpr syntax_option_type nosubs = unspecified; inline constexpr syntax_option_type optimize = unspecified; inline constexpr syntax_option_type collate = unspecified; inline constexpr syntax_option_type ECMAScript = unspecified; inline constexpr syntax_option_type basic = unspecified; inline constexpr syntax_option_type extended = unspecified; inline constexpr syntax_option_type awk = unspecified; inline constexpr syntax_option_type grep = unspecified; inline constexpr syntax_option_type egrep = unspecified; inline 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 113 .

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.

Table 113: syntax_option_type effects [tab:re.synopt]

🔗	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 object.
🔗	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]. See also: ECMA-262 15.10
🔗	basic	Specifies that the grammar recognized by the regular expression engine shall be that used by basic regular expressions in POSIX. See also: POSIX, Base Definitions and Headers, Section 9.3
🔗	extended	Specifies that the grammar recognized by the regular expression engine shall be that used by extended regular expressions in POSIX. See also: POSIX, Base Definitions and Headers, Section 9.4
🔗	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.

28.6.4.3 Bitmask type match_flag_type [re.matchflag]

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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; }

The type 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 114 for any bitmask elements set.

Table 114: regex_constants::match_flag_type effects [tab:re.matchflag]

🔗	Element	Effect(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.

28.6.4.4 Implementation-defined error_type [re.err]

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namespace std::regex_constants { using error_type = T3; inline constexpr error_type error_collate = unspecified; inline constexpr error_type error_ctype = unspecified; inline constexpr error_type error_escape = unspecified; inline constexpr error_type error_backref = unspecified; inline constexpr error_type error_brack = unspecified; inline constexpr error_type error_paren = unspecified; inline constexpr error_type error_brace = unspecified; inline constexpr error_type error_badbrace = unspecified; inline constexpr error_type error_range = unspecified; inline constexpr error_type error_space = unspecified; inline constexpr error_type error_badrepeat = unspecified; inline constexpr error_type error_complexity = unspecified; inline constexpr error_type error_stack = unspecified; }

The type error_type is an implementation-defined enumerated type ([enumerated.types]).

Values of type error_type represent the error conditions described in Table 115:

Table 115: error_type values in the C locale [tab:re.err]

🔗	Value	Error condition
🔗	error_collate	The expression contains an invalid collating element name.
🔗	error_ctype	The expression contains an invalid character class name.
🔗	error_escape	The expression contains an invalid escaped character, or a trailing escape.
🔗	error_backref	The expression contains an invalid back reference.
🔗	error_brack	The expression contains mismatched `[` and `]`.
🔗	error_paren	The expression contains mismatched `(` and `)`.
🔗	error_brace	The expression contains mismatched `{` and `}`
🔗	error_badbrace	The expression contains an invalid range in a `{}` expression.
🔗	error_range	The expression contains an invalid character range, such as `[b-a]` in most encodings.
🔗	error_space	There is insufficient memory to convert the expression into a finite state machine.
🔗	error_badrepeat	One of `*?+{` is not preceded by a valid regular expression.
🔗	error_complexity	The complexity of an attempted match against a regular expression exceeds a pre-set level.
🔗	error_stack	There is insufficient memory to determine whether the regular expression matches the specified character sequence.