std::regex_traits::value
From cppreference.com
                    
                                        
                    < cpp | regex | regex traits
                    
                                                            
                    |   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
| This section is incomplete Reason: no example  |