The expression "^group/([a-zA-Z0-9\_\.\-]+)/$" contains an unmatched '[' or ']'

When you convert these rewrite rules from an .htaccess file to a web.config file, you may encounter this error:
"The expression "^group/([a-zA-Z0-9\_\.\-]+)/$" contains an unmatched '[' or ']'." 
It was caused by the "_" symbol. In this condition, it is not necessary to escape "_" symbol, please try to use this pattern instead: