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#### Traits Class Requirements

There are two sets of requirements for the `traits` template argument to `basic_regex`: a minimal interface (which is part of the regex standardization proposal), and an optional Boost-specific enhanced interface.

##### Minimal requirements.

In the following table `X` denotes a traits class defining types and functions for the character container type `charT`; u is an object of type `X`; v is an object of type `const X`; p is a value of type `const charT*`; I1 and I2 are Input Iterators; c is a value of type `const charT`; s is an object of type `X::string_type`; cs is an object of type `const X::string_type`; b is a value of type `bool`; I is a value of type `int`; F1 and F2 are values of type `const charT*`; and loc is an object of type `X::locale_type`.

Expression | Return type | Assertion / Note Pre / Post condition
--- | --- | ---
X::char_type | charT | The character container type used in the implementation of class template basic_regex.
X::size_type |  | An unsigned integer type, capable of holding the length of a null-terminated string of charT's.
X::string_type | std::basic_string<charT> or std::vector<charT> |
X::locale_type | Implementation defined | A copy constructible type that represents the locale used by the traits class.
X::char_class_type | Implementation defined | A bitmask type representing a particular character classification. Multiple values of this type can be bitwise-or'ed together to obtain a new valid value.
X::length(p) | X::size_type | Yields the smallest i such that p[i] == 0. Complexity is linear in i.
v.translate(c) | X::char_type | Returns a character such that for any character d that is to be considered equivalent to c then v.translate(c) == v.translate(d).
v.translate_nocase(c) | X::char_type | For all characters C that are to be considered equivalent to c when comparisons are to be performed without regard to case, then v.translate_nocase(c) == v.translate_nocase(C).
v.transform(F1, F2) | X::string_type | 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).
v.transform_primary(F1, F2) | X::string_type | 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) when character case is not considered then v.transform_primary(G1, G2) < v.transform_primary(H1, H2).
v.lookup_classname(F1, F2) | X::char_class_type | Converts the character sequence designated by the iterator range [F1,F2) into a bitmask type that can subsequently be passed to isctype. Values returned from lookup_classname can be safely bitwise or'ed together. Returns 0 if the character sequence is not the name of a character class recognized by X. The value returned shall be independent of the case of the characters in the sequence.
v.lookup_collatename(F1, F2) | X::string_type | Returns a sequence of characters that represents the collating element consisting of the character sequence designated by the iterator range [F1, F2). Returns an empty string if the character sequence is not a valid collating element.
v.isctype(c, v.lookup_classname (F1, F2)) | bool | Returns true if character c is a member of the character class designated by the iterator range [F1, F2), false otherwise.
v.value(c, I) | int | Returns the value represented by the digit c in base I if the character c is a valid digit in base I; otherwise returns -1. [Note: the value of I will only be 8, 10, or 16. -end note]
u.imbue(loc) | X::locale_type | Imbues u with the locale loc, returns the previous locale used by u if any.
v.getloc() | X::locale_type | Returns the current locale used by v if any.

##### Additional Optional Requirements

The following additional requirements are strictly optional, however in order for `basic_regex` to take advantage of these additional interfaces, all of the following requirements must be met; `basic_regex` will detect the presence or absence of the member `boost_extensions_tag` and configure itself appropriately.

Expression | Result | Assertion / Note Pre / Post condition
--- | --- | ---
X::boost_extensions_tag | An unspecified type. | When present, all of the extensions listed in this table must be present.
v.syntax_type(c) | regex_constants::syntax_type | Returns a symbolic value of type regex_constants::syntax_type that signifies the meaning of character c within the regular expression grammar.
v.escape_syntax_type(c) | regex_constants::escape_syntax_type | Returns a symbolic value of type regex_constants::escape_syntax_type, that signifies the meaning of character c within the regular expression grammar, when c has been preceded by an escape character. Precondition: if b is the character preceding c in the expression being parsed then: `v.syntax_type(b) == syntax_escape`
v.translate(c, b) | X::char_type | Returns a character d such that: for any character d that is to be considered equivalent to c then `v.translate(c,false)==v.translate(d,false)`. Likewise for all characters C that are to be considered equivalent to c when comparisons are to be performed without regard to case, then `v.translate(c,true)==v.translate(C,true)`.
v.toi(I1, I2, i) | An integer type capable of holding either a charT or an int. | Behaves as follows: if `p == q` or if `*p` is not a digit character then returns -1. Otherwise performs formatted numeric input on the sequence [p,q) and returns the result as an int. Postcondition: either p == q or *p is a non-digit character.
v.error_string(I) | std::string | Returns a human readable error string for the error condition i, where i is one of the values enumerated by type regex_constants::error_type. If the value I is not recognized then returns the string "Unknown error" or a localized equivalent.
v.tolower(c) | X::char_type | Converts c to lower case, used for Perl-style \l and \L formatting operations.
v.toupper(c) | X::char_type | Converts c to upper case, used for Perl-style \u and \U formatting operations.