The blow code is replacing the SSN number with * from a sentence.

Sample Code:

String searchText = 'This is your SSN 000-45-6789 number';
Pattern objPattern = Pattern.compile('[0-9]{3}-[0-9]{2}-[0-9]{4}');
String result = objPattern.matcher(searchText).replaceAll('***-**-****');
System.debug('searchText-' + searchText);
System.debug('result-' + result);