Error displayed while trying to use an id prefixed by '$$' for autocomplete

Hi, I am trying to use autocomplete. It works well with input id's in plain text but gives an error mentioned below if my id is prefixed by $$

E.g

1:

<input id="tags" />

works well

2:

<input id="$$Cod_test" />

'Syntax error, unrecognized expression: #$$Cod_test'.

Kindly suggest.