std::regex_traits::value

From cppreference.com
int value( CharT ch, int radix ) const;

Determines the value represented by the digit ch in the numeric base radix, given the currently imbued locale.

[edit] Parameters

ch - the character that may represent a digit
radix - either 8, 10, or 16

[edit] Return value

The numeric value if ch indeed represents a digit in the currently imbued locale that is valid for the numeric base radix, or -1 on error.

[edit] Example