String st = str.replaceAll( "[\\n\\r]", "" );

Are you looking for code that will efficiently remove all \n and \r
characters in a string?

A flat-footed loop like this will do it faster than any regex.

private static string unwrap ( String s )
 {
 if ( s.indexOf( '\n' ) < 0 && s.indexOf( '\r' ) < 0 )
  {
  return s;
  }
 StringBuilder sb = new StringBuilder( s.length() );
 for ( int i=0; i<s.length(); i++ )
   {
   char c = s.charAt( i );
   if ( !( c == '\n' || c == '\r' ) )
      {
      sb.append( c );
      }
    }
 return s.toString();
 }
I think however, you actually probably want to replace \r\n with
space.

if ( c == '\n' || c == '\r' )
      {
      sb.append( ' ' );
      }
else
      {
      sb.append( c );
      }

If you want to use regexes, and replace different elements with
different expressions, you need to use

Matcher.appendReplacement and a StringBuffer.

to answer your question literally, all you need is a regex expression
that will match \n or \r as your first argument.  You could do that
with
"\\n|\\r" or "[\\n\\r]"