# std::basic_regex

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< cpp‎ | regex

Defined in header `<regex>` |  |
template < class CharT, class Traits = std::regex_traits<CharT> > class basic_regex; |  | (since C++11)

The class template `basic_regex` provides a general framework for holding regular expressions.

Several specializations for common character types are provided:

Defined in header `<regex>`
Type | Definition
`regex` | basic_regex<char>
`wregex` | basic_regex<wchar_t>

### Member types

Member type | Definition
`value_type` | CharT
`traits_type` | Traits
`string_type` | Traits::string_type
`locale_type` | Traits::locale_type
`flag_type` | std::regex_constants::syntax_option_type

### Member functions

(constructor) | constructs the regex object (public member function)
(destructor) | destructs the regex object (public member function)
operator= | assigns the contents (public member function)
assign | assigns the contents (public member function)

##### Observers

mark_count | returns the number of marked sub-expressions within the regular expression (public member function)
flags | returns the syntax flags (public member function)

##### Locale

getloc | get locale information (public member function)
imbue | set locale information (public member function)

##### Modifiers

swap | swaps the contents (public member function)

### Constants

Value | Effect(s)
`icase` | Character matching should be performed without regard to case.
`nosubs` | When performing matches, all marked sub-expressions `(expr)` are treated as non-marking sub-expressions `(?:expr)`. No matches are stored in the supplied std::regex_match structure and `mark_count()` is zero
`optimize` | Instructs the regular expression engine to make matching faster, with the potential cost of making construction slower. For example, this might mean converting a non-deterministic FSA to a deterministic FSA.
`collate` | Character ranges of the form "[a-b]" will be locale sensitive.
`multiline` (C++17) | Specifies that `^` shall match the beginning of a line and `$` shall match the end of a line, if the ECMAScript engine is selected.
`ECMAScript` | Use the Modified ECMAScript regular expression grammar
`basic` | Use the basic POSIX regular expression grammar (grammar documentation).
`extended` | Use the extended POSIX regular expression grammar (grammar documentation).
`awk` | Use the regular expression grammar used by the awk utility in POSIX (grammar documentation)
`grep` | Use the regular expression grammar used by the grep utility in POSIX. This is effectively the same as the `basic` option with the addition of newline '\n' as an alternation separator.
`egrep` | Use the regular expression grammar used by the grep utility, with the -E option, in POSIX. This is effectively the same as the `extended` option with the addition of newline '\n' as an alternation separator in addtion to '|'.

At most one grammar option must be chosen out of `ECMAScript`, `basic`, `extended`, `awk`, `grep`, `egrep`. If no grammar is chosen, `ECMAScript` is assumed to be selected. The other options serve as modifiers, such that std::regex("meow", std::regex::icase) is equivalent to std::regex("meow", std::regex::ECMAScript|std::regex::icase)

The member constants in `basic_regex` are duplicates of the syntax_option_type constants defined in the namespace `std::regex_constants`.

### Non-member functions

std::swap(std::basic_regex) (C++11) | specializes the std::swap algorithm (function template)

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