std::tolower
From cppreference.com
                    
                                        
                    
                    
                                                            
                    |   Defined in header <cctype>
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|   int tolower( int ch );  | 
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Converts the given character to lowercase.
[edit] Parameters
| ch | - | character to be converted | 
[edit] Return value
Lowercase version of ch or unmodified ch if no lowercase version is listed in the current C locale.
[edit] See also
|    converts a character to uppercase  (function)  | |
|    converts a character to lowercase using the ctype facet of a locale  (function template)  | |
|    converts a wide character to lowercase  (function)  | |
|   C documentation for tolower 
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