---
name: "std::regex_traits"
section: "3"
platform: "linux"
package: "gcc"
version: "16.2.1+r23+gd564253eb6c8-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/std::regex_traits"
---

# std::regex\_traits(3)

Describes aspects of a regular expression.

## NAME

std::regex\_traits\< \_Ch\_type \> - Describes aspects of a regular expression.

## SYNOPSIS

\#include \<regex\>

### Public Types

typedef \_RegexMask **char\_class\_type**\
 typedef \_Ch\_type **char\_type**\
 typedef **std::locale** **locale\_type**\
 typedef **std::basic\_string**\< char\_type \> **string\_type**\

### Public Member Functions

**regex\_traits** ()\
 Constructs a default traits object.  locale\_type **getloc** () const\
 Gets a copy of the current locale in use by the regex\_traits object.  locale\_type **imbue** (locale\_type \_\_loc)\
 Imbues the regex\_traits object with a copy of a new locale.  bool **isctype** (\_Ch\_type \_\_c, char\_class\_type \_\_f) const\
 Determines if c is a member of an identified class.  template\<typename \_Fwd\_iter\> char\_class\_type **lookup\_classname** (\_Fwd\_iter \_\_first, \_Fwd\_iter \_\_last, bool \_\_icase=false) const\
 Maps one or more characters to a named character classification.  template\<typename \_Fwd\_iter\> string\_type **lookup\_collatename** (\_Fwd\_iter \_\_first, \_Fwd\_iter \_\_last) const\
 Gets a collation element by name.  template\<typename \_Fwd\_iter\> string\_type **transform** (\_Fwd\_iter \_\_first, \_Fwd\_iter \_\_last) const\
 Gets a sort key for a character sequence.  template\<typename \_Fwd\_iter\> string\_type **transform\_primary** (\_Fwd\_iter \_\_first, \_Fwd\_iter \_\_last) const\
 Gets a sort key for a character sequence, independent of case.  char\_type **translate** (char\_type \_\_c) const\
 Performs the identity translation.  char\_type **translate\_nocase** (char\_type \_\_c) const\
 Translates a character into a case-insensitive equivalent.  int **value** (\_Ch\_type \_\_ch, int \_\_radix) const\
 Converts a digit to an int.

### Static Public Member Functions

static std::size\_t **length** (const char\_type \*\_\_p)\
 Gives the length of a C-style string starting at \_\_p.

### Protected Attributes

locale\_type **\_M\_locale**\

## Detailed Description

### template\<typename \_Ch\_type\>

class std::regex\_traits\< \_Ch\_type \>"Describes aspects of a regular expression.

A regular expression traits class that satisfies the requirements of section \[28.7\].

The class regex is parameterized around a set of related types and functions used to complete the definition of its semantics. This class satisfies the requirements of such a traits class.

**Since**

C++11

## Constructor & Destructor Documentation

### template\<typename \_Ch\_type\> std::regex\_traits\< \_Ch\_type \>::regex\_traits () \[inline\]

Constructs a default traits object.

Referenced by **value()**.

## Member Function Documentation

### template\<typename \_Ch\_type\> locale\_type std::regex\_traits\< \_Ch\_type \>::getloc () const \[inline\]

Gets a copy of the current locale in use by the regex\_traits object.

### template\<typename \_Ch\_type\> locale\_type std::regex\_traits\< \_Ch\_type \>::imbue (locale\_type \_\_loc) \[inline\]

Imbues the regex\_traits object with a copy of a new locale.

**Parameters**

*\_\_loc* A locale.

**Returns**

a copy of the previous locale in use by the regex\_traits object.

**Note**

Calling imbue with a different locale than the one currently in use invalidates all cached data held by \*this.

### template\<typename \_Ch\_type\> bool std::regex\_traits\< \_Ch\_type \>::isctype (\_Ch\_type \_\_c, char\_class\_type \_\_f) const

Determines if c is a member of an identified class.

**Parameters**

*\_\_c* a character. \
 *\_\_f* a class type (as returned from lookup\_classname).

**Returns**

true if the character \_\_c is a member of the classification represented by \_\_f, false otherwise.

**Exceptions**

**std::bad\_cast if the current locale does not have a ctype facet.**

**References std::use\_facet().**

### template\<typename \_Ch\_type\> std::size\_t std::regex\_traits\< \_Ch\_type \>::length (const char\_type \* \_\_p) \[inline\], \[static\]

**Gives the length of a C-style string starting at** \_\_p**.**

**Parameters**

*\_\_p* **a pointer to the start of a character sequence.**

**Returns**

**the number of characters between** \*\_\_p **and the first default-initialized value of type** char\_type**. In other words, uses the C-string algorithm for determining the length of a sequence of characters.**

### template\<typename \_Ch\_type\> template\<typename \_Fwd\_iter\> regex\_traits\< \_Ch\_type \>::char\_class\_type std::regex\_traits\< \_Ch\_type \>::lookup\_classname (\_Fwd\_iter \_\_first, \_Fwd\_iter \_\_last, bool \_\_icase = false) const

**Maps one or more characters to a named character classification.**

**Parameters**

*\_\_first* **beginning of the character sequence.** \
 *\_\_last* **one-past-the-end of the character sequence.** \
 *\_\_icase* **ignores the case of the classification name.**

**Returns**

**an unspecified value that represents the character classification named by the character sequence designated by the iterator range \[\_\_first, \_\_last). If** icase **is true, the returned mask identifies the classification regardless of the case of the characters to be matched (for example, \[\[:lower:\]\] is the same as \[\[:alpha:\]\]), otherwise a case-dependent classification is returned. The value returned shall be independent of the case of the characters in the character sequence. If the name is not recognized then returns a value that compares equal to 0.**

**At least the following names (or their wide-character equivalent) are supported.**

- •

  **d**

- •

  **w**

- •

  **s**

- •

  **alnum**

- •

  **alpha**

- •

  **blank**

- •

  **cntrl**

- •

  **digit**

- •

  **graph**

- •

  **lower**

- •

  **print**

- •

  **punct**

- •

  **space**

- •

  **upper**

- •

  **xdigit**

**References std::use\_facet().**

### template\<typename \_Ch\_type\> template\<typename \_Fwd\_iter\> regex\_traits\< \_Ch\_type \>::string\_type std::regex\_traits\< \_Ch\_type \>::lookup\_collatename (\_Fwd\_iter \_\_first, \_Fwd\_iter \_\_last) const

**Gets a collation element by name.**

**Parameters**

*\_\_first* **beginning of the collation element name.** \
 *\_\_last* **one-past-the-end of the collation element name.**

**Returns**

**a sequence of one or more characters that represents the collating element consisting of the character sequence designated by the iterator range \[\_\_first, \_\_last). Returns an empty string if the character sequence is not a valid collating element.**

**References std::basic\_string\< \_CharT, \_Traits, \_Alloc \>::empty(), and std::use\_facet().**

### template\<typename \_Ch\_type\> template\<typename \_Fwd\_iter\> string\_type std::regex\_traits\< \_Ch\_type \>::transform (\_Fwd\_iter \_\_first, \_Fwd\_iter \_\_last) const \[inline\]

**Gets a sort key for a character sequence.**

**Parameters**

*\_\_first* **beginning of the character sequence.** \
 *\_\_last* **one-past-the-end of the character sequence.**

**Returns a sort key for the character sequence designated by the iterator range \[F1, F2) such that if the character sequence \[G1, G2) sorts before the character sequence \[H1, H2) then v.transform(G1, G2) \< v.transform(H1, H2).**

**What this really does is provide a more efficient way to compare a string to multiple other strings in locales with fancy collation rules and equivalence classes.**

**Returns**

**a locale-specific sort key equivalent to the input range.**

**Exceptions**

**std::bad\_cast if the current locale does not have a collate facet.**

### template\<typename \_Ch\_type\> template\<typename \_Fwd\_iter\> string\_type std::regex\_traits\< \_Ch\_type \>::transform\_primary (\_Fwd\_iter \_\_first, \_Fwd\_iter \_\_last) const \[inline\]

**Gets a sort key for a character sequence, independent of case.**

**Parameters**

*\_\_first* **beginning of the character sequence.** \
 *\_\_last* **one-past-the-end of the character sequence.**

**Effects: if** typeid(use\_facet\<collate\<\_Ch\_type\>\>(getloc())) == typeid(collate\_byname\<\_Ch\_type\>) **and the form of the sort key returned by** collate\_byname\<\_Ch\_type\>::transform(\_\_first, \_\_last) **is known and can be converted into a primary sort key then returns that key, otherwise returns an empty string.**

**Todo**

**Implement this function correctly.**

### template\<typename \_Ch\_type\> char\_type std::regex\_traits\< \_Ch\_type \>::translate (char\_type \_\_c) const \[inline\]

**Performs the identity translation.**

**Parameters**

*\_\_c* **A character to the locale-specific character set.**

**Returns**

**\_\_c.**

### template\<typename \_Ch\_type\> char\_type std::regex\_traits\< \_Ch\_type \>::translate\_nocase (char\_type \_\_c) const \[inline\]

**Translates a character into a case-insensitive equivalent.**

**Parameters**

*\_\_c* **A character to the locale-specific character set.**

**Returns**

**the locale-specific lower-case equivalent of \_\_c.**

**Exceptions**

**std::bad\_cast if the imbued locale does not support the ctype facet.**

### template\<typename \_Ch\_type\> int std::regex\_traits\< \_Ch\_type \>::value (\_Ch\_type \_\_ch, int \_\_radix) const

**Converts a digit to an int.**

**Parameters**

*\_\_ch* **a character representing a digit.** \
 *\_\_radix* **the radix if the numeric conversion (limited to 8, 10, or 16).**

**Returns**

**the value represented by the digit \_\_ch in base radix if the character \_\_ch is a valid digit in base radix; otherwise returns -1.**

**References regex\_traits(), and std::use\_facet().**

## Author

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